Ipac logo

Publications

The List of ZTF Papers from the astronomical community:

 The Full ZTF-Publication ADS library (a total of 210 papers as of Nov 4th 2020)

 

Below is the list of ZTF papers published by authors from Caltech and the ZTF partners:
95. A Mini-Neptune and a Radius Valley Planet Orbiting the Nearby M2 Dwarf TOI-1266 in Its Venus Zone: Validation with the Habitable-zone Planet Finder
Authors
Stefánsson, Guðmundur, Kopparapu, Ravi, Lin, Andrea, Mahadevan, Suvrath, Cañas, Caleb I., Kanodia, Shubham, Ninan, Joe P., Cochran, William D., Endl, Michael, Hebb, Leslie, Wisniewski, John, Gupta, Arvind, Everett, Mark, Bender, Chad F., Diddams, Scott A., Ford, Eric B., Fredrick, Connor, Halverson, Samuel, Hearty, Fred, Levi, Eric, Maney, Marissa, Metcalf, Andrew J., Monson, Andrew, Ramsey, Lawrence W., Robertson, Paul, Roy, Arpita, Schwab, Christian, Terrien, Ryan C., Wright, Jason T.

We report on the validation of two planets orbiting the nearby (36 pc) M2 dwarf TOI-1266 observed by the TESS mission. This system is one of a few M dwarf multiplanet systems with close-in planets where the inner planet is substantially larger than the outer planet. The inner planet is sub-Neptune-sized (R = 2.46 ± 0.08 R_⊕) with an orbital period of 10.9 days, while the outer planet has a radius of 1.67_(-0.11)^(+0.09) R_⊕ and resides in the exoplanet radius valley—the transition region between rocky and gaseous planets. With an orbital period of 18.8 days, the outer planet receives an insolation flux of 2.4 times that of Earth, similar to the insolation of Venus. Using precision near-infrared radial velocities with the Habitable-zone Planet Finder Spectrograph, we place upper mass limits of 15.9 and 6.4 M_⊕ at 95% confidence for the inner and outer planet, respectively. A more precise mass constraint of both planets, achievable with current radial velocity instruments given the host star brightness (V = 12.9, J = 9.7), will yield further insights into the dominant processes sculpting the exoplanet radius valley.

Status
Published
Date
November 16th, 2020
Article doi
https://doi.org/10.3847/1538-3881/abbe19
Discussion Paper arxiv
https://arxiv.org/abs/2006.11180
94. An 8.8 minute orbital period eclipsing detached double white dwarf binary
Authors
Burdge, Kevin B., Coughlin, Michael W., Fuller, Jim, Kaplan, David L., Kulkarni, S. R., Marsh, Thomas R., Prince, Thomas A., Bellm, Eric C., Dekany, Richard G., Duev, Dmitry A., Graham, Matthew J., Mahabal, Ashish A., Masci, Frank J., Laher, Russ R., Riddle, Reed, Soumagnac, Maayane T.

We report the discovery of ZTF J2243+5242, an eclipsing double white dwarf binary with an orbital period of just 8.8 minutes, the second known eclipsing binary with an orbital period less than ten minutes. The system likely consists of two low-mass white dwarfs, and will merge in approximately 400,000 years to form either an isolated hot subdwarf or an R Coronae Borealis star. Like its 6.91min counterpart, ZTF J1539+5027, ZTF J2243+5242 will be among the strongest gravitational wave sources detectable by the space-based gravitational-wave detector The Laser Space Interferometer Antenna (LISA) because its gravitational-wave frequency falls near the peak of LISA's sensitivity. Based on its estimated distance of d = 2120⁺¹³¹₋₁₁₅pc, LISA should detect the source within its first few months of operation, and should achieve a signal-to-noise ratio of 87±5 after four years. We find component masses of M_A = 0.349^(+0.093)_(−0.074)M⊙ and M_B = 0.384^(+0.114)_(−0.074)M⊙, radii of R_A = 0.0308^(+0.0026)_(−0.0025)R⊙ and R_B = 0.0291^(+0.0032)_(−0.0024)R⊙, and effective temperatures of T_A = 22200⁺¹⁸⁰⁰₋₁₆₀₀K and T_B = 16200⁺¹²⁰⁰₋₁₀₀₀K. We determined all of these properties, and the distance to this system, using only photometric measurements, demonstrating a feasible way to estimate parameters for the large population of optically faint (r > 21m_(AB)) gravitational-wave sources which the Vera Rubin Observatory (VRO) and LISA should identify.

Status
Unpublished
Date
November 4th, 2020
Discussion Paper arxiv
http://arxiv.org/abs/2010.03555
93. Zwicky Transient Facility Observations of Trojan Asteroids: A Thousand Colors, Rotation Amplitudes, and Phase Functions
Authors
Schemel, Madeline, Brown, Michael E.

We introduce a new method for analyzing sparse photometric data of asteroids and apply it to Zwicky Transient Facility observations of the Jupiter Trojan asteroids. The method relies on the creation of a likelihood model that includes the probability distribution of rotational brightness variations at an unknown rotation phase. The likelihood model is analyzed via a Markov Chain Monte Carlo to quantify the uncertainty in our parameter estimates. Using this method, we provide color, phase parameter, absolute magnitude and amplitude of rotation measurements for 1049 Jupiter Trojans. We find that phase parameter is correlated with color and the distribution of Trojan asteroid rotational amplitudes is indistinguishable from that of main-belt asteroids.

Status
Submitted
Date
November 4th, 2020
Discussion Paper arxiv
http://arxiv.org/abs/2011.01329
92. Periodic Eruptive Variability of the Isolated Pre-main-sequence Star V347 Aurigae
Authors
Dahm, S. E., Hillenbrand, L. A.

V347 Aurigae is associated with the small dark cloud L1438 and appears to be an isolated pre-main-sequence star located at distance d ≈ 200 pc. Multi-epoch, archival photometry reveals periodic brightness variations with amplitude V ≈ 2.0 mag occurring on timescales of ~160 days that have persisted for decades. Regular-cadence, optical imaging of the source with the Zwicky Transient Facility shows that a small reflection nebula illuminated by V347 Aur also fluctuates in brightness, at times fading completely. Multi-epoch, Keck/HIRES data suggests the presence of two distinct spectral components: a prominent emission-line-dominated spectrum with a heavily veiled continuum correlated with the bright photometric state, and an M-type absorption line spectrum associated with quiescence. All spectra exhibit strong Balmer and He i line emission, consistent with accretion, as well as high velocity emission arising from the forbidden transitions of [O i], [N ii], and [S ii] that are generally associated with collimated jets and disk winds. There is no evidence in existing high-dispersion spectroscopy or high-resolution imaging for binarity of V347 Aur. The repeating outburst events are possibly linked to accretion instabilities induced by an undetected companion or a structure within the circumstellar disk that periodically increases the mass accretion rate. V347 Aur is perhaps analogous to an EXor-type variable, though more regularly recurring.

Status
Published
Date
October 12th, 2020
Article doi
https://doi.org/10.3847/1538-3881/abbfa2
Discussion Paper arxiv
https://arxiv.org/abs/2010.02996
91. The First Extensive Spectroscopic Study of Young Stars in the North America and Pelican Nebulae Region
Authors
Fang, Min, Hillenbrand, Lynne A., Kim, Jinyoung Serena, Findeisen, Krzysztof, Herczeg, Gregory J., Carpenter, John M., Rebull, Luisa M., Wang, Hongchi

We present a spectroscopic survey of over 3400 potential members in the North America and Pelican nebulae (NAP) using several low-resolution (R≈ 1300-2000) spectrographs: Palomar/Norris, WIYN/HYDRA, Keck/DEIMOS, and MMT/Hectospec. We identify 580 young stars as likely members of the NAP region based on criteria involving infrared excess, Li I 6708 absorption, X-ray emission, parallax, and proper motions. The spectral types of individual spectra are derived by fitting them with templates that are either empirical spectra of pre-main sequence stars, or model atmospheres. The templates are artificially veiled, and a best-fit combination of spectral type and veiling parameter is derived for each star. We use the spectral types with archival photometry to derive V-band extinction and stellar luminosity. From the H-R diagram, the median age of the young stars is about 1 Myr, with a luminosity dispersion of ∼0.3--0.4 dex. We investigate the photometric variability of the spectroscopic member sample using ZTF data, and conclude that photometric variability, while present, does not significantly contribute to the luminosity dispersion. While larger than the formal errors, the luminosity dispersion is smaller than if veiling were not taken into account in our spectral typing process. The measured ages of stellar kinematic groups, combined with inferred ages for embedded stellar populations revealed by Spitzer, suggests a sequential history of star formation in the NAP region.

Status
Unpublished
Date
October 9th, 2020
Discussion Paper arxiv
http://arxiv.org/abs/2009.11995v1
90. SN 2020bqj: a Type Ibn supernova with a long lasting peak plateau
Authors
Kool, E. C., Karamehmetoglu, E., Sollerman, J., Schulze, S., Lunnan, R., Reynolds, T. M., Barbarino, C., Bellm, E. C., De, K., Duev, D. A., Fremling, C., Golkhou, V. Z., Graham, M. L., Green, D. A., Horesh, A., Kaye, S., Kim, Y.-L., Laher, R. R., Masci, F. J., Nordin, J., Perley, D. A., Phinney, E. S., Porter, M., Reiley, D., Rodriguez, H., van Roestel, J., Rusholme, B., Sharma, Y., Sfaradi, I., Soumagnac, M. T., Taggart, K., Tartaglia, L., Williams, D. R. A., Yan, L.

Context. Type Ibn supernovae (SNe Ibn) are a rare class of stripped envelope supernovae interacting with a helium-rich circumstellar medium (CSM). The majority of the SNe Ibn reported in the literature display a surprising homogeneity in their fast-evolving lightcurves and are typically found in actively starforming spiral galaxies. Aims. We present the discovery and the study of SN 2020bqj (ZTF20aalrqbu), a SN Ibn with a long-duration peak plateau lasting 40 days and hosted by a faint low-mass galaxy. We aim to explain its peculiar properties using an extensive photometric and spectroscopic data set. Methods. We compare the photometric and spectral evolution of SN 2020bqj with regular SNe Ibn from the literature, as well as with other outliers in the SN Ibn subclass. We fit the bolometric and multi-band lightcurves with powering mechanism models such as radioactive decay and CSM interaction. We also model the host galaxy of SN 2020bqj. Results. The risetime, peak magnitude and spectral features of SN 2020bqj are consistent with those of most SNe Ibn, but the SN is a clear outlier in the subclass based on its bright, long-lasting peak plateau and the low mass of its faint host galaxy. We show through modeling that the lightcurve of SN 2020bqj can be powered predominantly by shock heating from the interaction of the SN ejecta and a dense CSM. The peculiar Type Ibn SN 2011hw is a close analog to SN 2020bqj in terms of lightcurve and spectral evolution, suggesting a similar progenitor and CSM scenario. In this scenario a very massive progenitor star in the transitional phase between a luminous blue variable and a compact Wolf-Rayet star undergoes core-collapse, embedded in a dense helium-rich CSM with an elevated opacity compared to normal SNe Ibn, due to the presence of residual hydrogen. This scenario is consistent with the observed properties of SN 2020bqj and the modeling results.

Status
Unpublished
Date
September 17th, 2020
Discussion Paper arxiv
http://arxiv.org/abs/2008.04056
89. A large fraction of hydrogen-rich supernova progenitors experience elevated mass loss shortly prior to explosion
Authors
Bruch, Rachel J., Gal-Yam, Avishay, Schulze, Steve, Yaron, Ofer, Yang, Yi, Soumagnac, Maayane T., Rigault, Mickael, Strotjohann, Nora L., Ofek, Eran O., Sollerman, Jesper, Masci, Frank J., Barbarino, Cristina, Ho, Anna Y. Q., Fremling, Christoffer, Perley, Daniel A., Nordin, Jakob, Cenko, S. Bradley, Adams, S., Adreoni, Igor, Bellm, Eric C., Blagorodnova, Nadia, Bulla, Mattia, Burdge, Kevin, De, Kishalay, Dhawan, Suhail, Drake, Andrew J., Duev, Dmitry A., Dugas, Alison, Graham, Matthew, Graham, Melissa L., Jencson, Jacob, Karamehmetoglu, Emir, Kasliwal, Mansi, Kim, Young-Lo, Kulkarni, Shrinivas, Kupfer, Thomas, Mahabal, Ashish, Miller, A. A., Prince, Thomas A., Riddle, Reed, Sharma, Y., Smith, Roger, Taddia, Francesco, Taggart, Kirsty, Walters, Richard, Yan, Lin

Spectroscopic detection of narrow emission lines traces the presence of circumstellar mass distributions around massive stars exploding as core-collapse supernovae. Transient emission lines disappearing shortly after the supernova explosion suggest that the spatial extent of such material is compact, and hence imply an increased mass loss shortly prior to explosion. Here, we present a systematic survey for such transient emission lines (Flash Spectroscopy) among Type II supernovae detected in the first year of the Zwicky Transient Facility (ZTF) survey. We find that at least six out of ten events for which a spectrum was obtained within two days of estimated explosion time show evidence for such transient flash lines. Our measured flash event fraction (>30% at 95% confidence level) indicates that elevated mass loss is a common process occurring in massive stars that are about to explode as supernovae.

Status
Unpublished
Date
September 17th, 2020
Discussion Paper arxiv
http://arxiv.org/abs/2008.09986
88. A Warm Jupiter Transiting an M Dwarf: A TESS Single-transit Event Confirmed with the Habitable-zone Planet Finder
Authors
Cañas, Caleb I., Stefansson, Gudmundur, Kanodia, Shubham, Mahadevan, Suvrath, Cochran, William D., Endl, Michael, Robertson, Paul, Bender, Chad F., Ninan, Joe P., Beard, Corey, Lubin, Jack, Gupta, Arvind F., Everett, Mark E., Monson, Andrew, Wilson, Robert F., Lewis, Hannah M., Brewer, Mary, Majewski, Steven R., Hebb, Leslie, Dawson, Rebekah I., Diddams, Scott A., Ford, Eric B., Fredrick, Connor, Halverson, Samuel, Hearty, Fred, Lin, Andrea S. J., Metcalf, Andrew J., Rajagopal, Jayadev, Ramsey, Lawrence W., Roy, Arpita, Schwab, Christian, Terrien, Ryan C., Wright, Jason T.

We confirm the planetary nature of a warm Jupiter transiting the early M dwarf TOI-1899 using a combination of available TESS photometry; high-precision, near-infrared spectroscopy with the Habitable-zone Planet Finder; and speckle and adaptive optics imaging. The data reveal a transiting companion on an ~29 day orbit with a mass and radius of 0.66 ± 0.07 M_J and 1.15_(-0.05)^(+0.04) R_J, respectively. The star, TOI-1899, is the lowest-mass star known to host a transiting warm Jupiter, and we discuss the follow-up opportunities afforded by a warm (T_(eq) ~ 362 K) gas giant orbiting an M0 star. Our observations reveal that TOI-1899.01 is a puffy warm Jupiter, and we suggest additional transit observations to both refine the orbit and constrain the true dilution observed in TESS.

Status
Published
Date
September 17th, 2020
Article doi
https://doi.org/10.3847/1538-3881/abac67
Discussion Paper arxiv
https://arxiv.org/abs/2007.07098
87. A Non-equipartition Shock Wave Traveling in a Dense Circumstellar Environment around SN 2020oi
Authors
Horesh, Assaf, Sfaradi, Itai, Ergon, Mattias, Barbarino, Cristina, Sollerman, Jesper, Moldon, Javier, Dobie, Dougal, Schulze, Steve, Pérez-Torres, Miguel, Williams, David R. A., Fremling, Christoffer, Gal-Yam, Avishay, Kulkarni, Shrinivas R., O'Brien, Andrew, Lundqvist, Peter, Murphy, Tara, Fender, Rob, Anand, Shreya, Belicki, Justin, Bellm, Eric C., Coughlin, Michael W., De, Kishalay, Ofek, Eran O., Golkhou, V. Zach, Graham, Matthew J., Green, Dave A., Hankins, Matthew, Kasliwal, Mansi M., Kupfer, Thomas, Laher, Russ R., Masci, Frank J., Miller, Adam A., Neill, James D., Perrott, Yvette, Porter, Michael, Reiley, Daniel J., Rigault, Mickael, Rodriguez, Hector, Rusholme, Ben, Shupe, David L., Titterington, David

We report the discovery and panchromatic follow-up observations of the young Type Ic supernova (SN Ic) SN 2020oi in M100, a grand-design spiral galaxy at a mere distance of 14 Mpc. We followed up with observations at radio, X-ray, and optical wavelengths from only a few days to several months after explosion. The optical behavior of the supernova is similar to those of other normal SNe Ic. The event was not detected in the X-ray band but our radio observations revealed a bright mJy source (L_ν ≈ 1.2 × 10²⁷ ergs⁻¹ Hz⁻¹). Given the relatively small number of stripped envelope SNe for which radio emission is detectable, we used this opportunity to perform a detailed analysis of the comprehensive radio data set we obtained. The radio-emitting electrons initially experience a phase of inverse Compton cooling, which leads to steepening of the spectral index of the radio emission. Our analysis of the cooling frequency points to a large deviation from equipartition at the level of ϵ_e/ϵ_B ≳ 200, similar to a few other cases of stripped envelope SNe. Our modeling of the radio data suggests that the shock wave driven by the SN ejecta into the circumstellar matter (CSM) is moving at ∼3 × 10⁴ km s⁻¹. Assuming a constant mass loss from the stellar progenitor, we find that the mass-loss rate is Ṁ ≈ 1.4 × 10⁻⁴ M_⊙ yr⁻¹ for an assumed wind velocity of 1000 km s⁻¹. The temporal evolution of the radio emission suggests a radial CSM density structure steeper than the standard r⁻².

Status
Published
Date
September 17th, 2020
Article doi
https://doi.org/10.3847/1538-4357/abbd38
Discussion Paper arxiv
https://arxiv.org/abs/2006.13952
86. The Zwicky Transient Facility Bright Transient Survey. II. A Public Statistical Sample for Exploring Supernova Demographics
Authors
Perley, Daniel A., Fremling, Christoffer, Sollerman, Jesper, Miller, Adam A., Dahiwale, Aishwarya S., Sharma, Yashvi, Bellm, Eric C., Biswas, Rahul, Brink, Thomas G., Bruch, Rachel J., De, Kishalay, Dekany, Richard, Drake, Andrew J., Duev, Dmitry A., Filippenko, Alexei V., Gal-Yam, Avishay, Goobar, Ariel, Graham, Matthew J., Graham, Melissa L., Ho, Anna Y. Q., Irani, Ido, Kasliwal, Mansi M., Kim, Young-Lo, Kulkarni, S. R., Mahabal, Ashish, Masci, Frank J., Modak, Shaunak, Neill, James D., Nordin, Jakob, Riddle, Reed L., Soumagnac, Maayane T., Strotjohann, Nora L., Schulze, Steve, Taggart, Kirsty, Tzanidakis, Anastasios, Walters, Richard, Yan, Lin

We present a public catalog of transients from the Zwicky Transient Facility (ZTF) Bright Transient Survey, a magnitude-limited (m < 19 mag in either the g or r filter) survey for extragalactic transients in the ZTF public stream. We introduce cuts on survey coverage, sky visibility around peak light, and other properties unconnected to the nature of the transient, and show that the resulting statistical sample is spectroscopically 97% complete at <18 mag, 93% complete at <18.5 mag, and 75% complete at <19 mag. We summarize the fundamental properties of this population, identifying distinct duration–luminosity correlations in a variety of supernova (SN) classes and associating the majority of fast optical transients with well-established spectroscopic SN types (primarily SN Ibn and II/IIb). We measure the Type Ia SN and core-collapse (CC) SN rates and luminosity functions, which show good consistency with recent work. About 7% of CC SNe explode in very low-luminosity galaxies (M_i > −16 mag), 10% in red-sequence galaxies, and 1% in massive ellipticals. We find no significant difference in the luminosity or color distributions between the host galaxies of SNe Type II and SNe Type Ib/c, suggesting that line-driven wind stripping does not play a major role in the loss of the hydrogen envelope from their progenitors. Future large-scale classification efforts with ZTF and other wide-area surveys will provide high-quality measurements of the rates, properties, and environments of all known types of optical transients and limits on the existence of theoretically predicted but as yet unobserved explosions.

Status
Published
Date
September 17th, 2020
Article doi
https://doi.org/10.3847/1538-4357/abbd98
Discussion Paper arxiv
https://arxiv.org/abs/2009.01242
Intearctive data catalog related
https://www.astro.caltech.edu/ztf/bts
85. A systematic search of Zwicky Transient Facility data for ultracompact binary LISA-detectable gravitational-wave sources
Authors
Burdge, Kevin B., Prince, Thomas A., Fuller, Jim, Kaplan, David L., Marsh, Thomas R., Tremblay, Pier-Emmanuel, Zhuang, Zhuyun, Bellm, Eric C., Caiazzo, Ilaria, Coughlin, Michael W., Dhillon, Vik S., Gaensicke, Boris, Rodríguez-Gil, Pablo, Graham, Matthew J., Hermes, J. J., Kupfer, Thomas, Littlefair, S. P., Mróz, Przemek, Phinney, E. S., van Roestel, Jan, Yao, Yuhan, Dekany, Richard G., Drake, Andrew J., Duev, Dmitry A., Hale, David, Feeney, Michael, Helou, George, Kaye, Stephen, Mahabal, Ashish A., Masci, Frank J., Riddle, Reed, Smith, Roger, Soumagnac, Maayane T., Kulkarni, S. R.

Using photometry collected with the Zwicky Transient Facility (ZTF), we are conducting an ongoing survey for binary systems with short orbital periods (P_b < 1 hr) with the goal of identifying new gravitational-wave sources detectable by the upcoming Laser Interferometer Space Antenna (LISA). Here, we present a sample of fifteen binary systems discovered thus far, with orbital periods ranging from 6.91 min to 56.35 min. Of the fifteen systems, seven are eclipsing systems which do not show signs of significant mass transfer. Additionally, we have discovered two AM Canum Venaticorum (AM CVn) systems and six systems exhibiting primarily ellipsoidal variations in their light curves. We present follow-up spectroscopy and high-speed photometry confirming the nature of these systems, estimates of their LISA signal-to-noise ratios (SNR), and a discussion of their physical characteristics.

Status
Unpublished
Date
September 16th, 2020
Discussion Paper arxiv
http://arxiv.org/abs/2009.02567
84. SN 2018ijp: the explosion of a stripped-envelope star within a dense H-rich shell?
Authors
Tartaglia, L., Sollerman, J., Barbarino, C., Taddia, F., Mason, E., Berton, M., Taggart, K., Bellm, E. C., De, K., Frederick, S., Fremling, C., Gal-Yam, A., Golkhou, V. Z., Graham, M., Ho, A. Y. Q., Hung, T., Kaye, S., Kim, Y. L., Laher, R. R., Masci, F. J., Perley, D. A., Porter, M. D., Reiley, D. J., Riddle, R., Rusholme, B., Soumagnac, M. T., Walters, R.

In this paper, we discuss the outcomes of the follow-up campaign of SN 2018ijp, discovered by the Zwicky Transient Facility survey for optical transients. SN 2018ijp shows early spectra similar to broad-lined supernovae of Type Ic around maximum light, followed later by strong signatures of interaction between rapidly expanding supernova ejecta and a dense H-rich circumstellar medium, coinciding with a second peak in the photometric evolution of the transient. Modeling the early luminosity of SN 2018ijp results in 0.7 M_⊙ and 0.3 M_⊙ for the total ejected and radioactive ⁵⁶Ni masses, with an explosion energy of 3.3 × 10⁵¹ erg, while the analysis of the light curve at later phases suggests a total mass of ≃0.5 M_⊙ for the H-rich circumstellar medium. Based on these results, obtained using simple analytical models, we discuss the observables of SN 2018jp in the context of the explosion of a massive star depleted of its outer H and He layers within a dense H-rich medium.

Status
Unpublished
Date
September 16th, 2020
Discussion Paper arxiv
http://arxiv.org/abs/2009.03331
83. A kilometer-scale asteroid inside Venus's orbit
Authors
Ip, W.-H., Bolin, B., Masci, F. J., Ye, Q.-Z., Kramer, E. A., Helou, G., Ahumada, T., Coughlin, M. W., Graham, M. J., Walters, R., Deshmukh, K. P., Fremling, C., Lin, Z.-Y., Milburn, J. W., Purdum, J. N., Quimby, R., Bodewits, D., Chang, C.-K., Ngeow, C.-C., Tan, H., Zhai, C., van Dokkum, P., Granvik, M., Harikane, Y., Mowla, L. A., Burdge, K. B., Bellm, E. C., De, K., Cenko, S. B., Copperwheat, C. M., Dekany, R., Duev, D. A., Hale, D., Kasliwal, M. M., Kulkarni, S. R., Kupfer, T., Mahabal, A., Mróz, P. J., Neill, J. D., Riddle, R., Rodriguez, H., Serabyn, E., Smith, R. M., Sollerman, J., Soumagnac, M. T., Southworth, J., Yan, L.

Near-Earth asteroid population models predict the existence of asteroids located inside the orbit of Venus. However, despite searches up to the end of 2019, none have been found. Here we report the discovery by the Zwicky Transient Facility of the first known asteroid located inside of Venus' orbit, 2020 AV₂, possessing an aphelion distance of 0.65 au and ∼2 km in size. While it is possible that 2020 AV₂ is the largest of its kind, we find that its discovery is surprising in the context of population models where the expected count is close to zero. If this discovery is not a statistical fluke, then 2020 AV₂ may come from a yet undiscovered source population of asteroids interior to Venus, and currently favored asteroid population models may need to be adjusted.

Status
Unpublished
Date
September 16th, 2020
Discussion Paper arxiv
http://arxiv.org/abs/2009.04125
82. Two stripped envelope supernovae with circumstellar interaction. But only one really shows it
Authors
Sollerman, J., Fransson, C., Barbarino, C., Fremling, C., Horesh, A., Kool, E., Schulze, S., Sfaradi, I., Yang, S., Bellm, E. C., Burruss, R., Cunningham, V., De, K., Drake, A. J., Golkhou, V. Z., Green, D. A., Kasliwal, M., Kulkarni, S., Kupfer, T., Laher, R. R., Masci, F. J., Rodriguez, H., Rusholme, B., Williams, D. R. A., Yan, L., Zolkower, J.

Context. We present observations of SN 2019tsf (ZTF19ackjszs) and SN 2019oys (ZTF19abucwzt). These two stripped envelope (SE) Type Ib supernovae (SNe) suddenly showed a (re-)brightening in their late light curves. We investigate this in the context of circumstellar material (CSM) interaction with previously ejected material, a phenomenon that is unusual among SE SNe. Aims. We use our follow-up photometry and spectroscopy for these supernovae to demonstrate the presence of CSM interaction, estimate the properties of the CSM, and discuss why the signals are so different for the two objects. Methods. We present and analyze observational data, consisting of optical light curves and spectra. For SN 2019oys, we also have detections in radio as well as limits from UV and X-rays. Results. Both light curves show spectacular re-brightening after about 100 days. In the case of SN 2019tsf, the re-brightening is followed by a new period of decline, and the spectra never show signs of narrow emission lines that would indicate CSM interaction. On the contrary, SN 2019oys made a spectral makeover from a Type Ib to a spectrum clearly dominated by CSM interaction at the light curve brightening phase. Deep Keck spectra reveal a plethora of narrow high-ionization lines, including coronal lines, and the radio observations show strong emission. Conclusions. The rather similar light curve behavior – with a late linear re-brightening – of these two Type Ib SE SNe indicate CSM interaction as the powering source. For SN 2019oys the evidence for a phase where the ejecta hit H-rich material, likely ejected from the progenitor star, is conspicuous. We observe strong narrow lines of H and He, but also a plethora of high-ionization lines, including coronal lines, revealing shock interaction. Spectral simulations of SN 2019oys show two distinct density components, one with density ≳10⁹ cm⁻³, dominated by somewhat broader, low-ionization lines of H I, He I, Na I, and Ca II, and one with narrow, high-ionization lines at a density ∼10⁶ cm⁻³. The former is strongly affected by electron scattering, while the latter is unaffected. The evidence for CSM interaction in SN 2019oys is corroborated by detections in radio. On the contrary, for SN 2019tsf, we find little evidence in the spectra for any CSM interaction.

Status
Published
Date
September 16th, 2020
Article doi
https://doi.org/10.1051/0004-6361/202038960
Discussion Paper arxiv
https://arxiv.org/abs/2009.04154
81. Constraining the Kilonova Rate with Zwicky Transient Facility Searches Independent of Gravitational Wave and Short Gamma-ray Burst Triggers
Authors
Andreoni, Igor, Kool, Erik C., Sagués Carracedo, Ana, Kasliwal, Mansi M., Bulla, Mattia, Ahumada, Tomás, Coughlin, Michael W., Anand, Shreya, Sollerman, Jesper, Goobar, Ariel, Kaplan, David L., Loveridge, Tegan T., Karambelkar, Viraj, Cooke, Jeff, Bagdasaryan, Ashot, Bellm, Eric C., Cenko, S. Bradley, Cook, David O., De, Kishalay, Dekany, Richard, Delacroix, Alexandre, Drake, Andrew, Duev, Dmitry A., Fremling, Christoffer, Golkhou, V. Zach, Graham, Matthew J., Hale, David, Kulkarni, S. R., Kupfer, Thomas, Laher, Russ R., Mahabal, Ashish A., Masci, Frank J., Rusholme, Ben, Smith, Roger M., Tzanidakis, Anastasios, Van Sistine, Angela, Yao, Yuhan

The first binary neutron star merger, GW170817, was accompanied by a radioactivity-powered optical/infrared transient called a kilonova. To date, no compelling kilonova has been found during optical surveys of the sky, independent of gravitational-wave triggers. In this work, we searched the first 23 months of the Zwicky Transient Facility (ZTF) data stream for candidate kilonovae in the form of rapidly evolving transients. We combined ZTF alert queries with forced point-spread-function photometry and nightly flux stacking to increase our sensitivity to faint and fast transients. Automatic queries yielded > 11,200 candidates, 24 of which passed quality checks and strict selection criteria based on a grid of kilonova models tailored for both binary neutron star and neutron star-black hole mergers. None of the candidates in our sample was deemed a possible kilonova after thorough vetting, catalog cross-matching, and study of their color evolution. The sources that passed our selection criteria are dominated by Galactic cataclysmic variables. In addition, we identified two fast transients at high Galactic latitude, one of which is the confirmed afterglow of long-duration GRB190106A, and the other is a possible cosmological afterglow. Using a survey simulation code, we constrained the kilonova rate for a range of models including top-hat and linearly decaying light curves and synthetic light curves obtained with radiative transfer simulations. For prototypical GW170817-like kilonovae, we constrain the rate to be R < 1775 Gpc⁻³ yr⁻¹ at 95% confidence level by requiring at least 2 high-significance detections. By assuming a population of kilonovae with the same geometry and composition of GW170817 observed under a uniform viewing angle distribution, we obtained a constraint on the rate of R < 4029 Gpc⁻³ yr⁻¹.

Status
Unpublished
Date
September 8th, 2020
Discussion Paper arxiv
http://arxiv.org/abs/2008.00008
80. Characterization of Temporarily Captured Minimoon 2020 CD₃ by Keck Time-resolved Spectrophotometry
Authors
Bolin, Bryce T., Fremling, Christoffer, Holt, Timothy R., Hankins, Matthew J., Ahumada, Tomás, Anand, Shreya, Bhalerao, Varun, Burdge, Kevin B., Copperwheat, Chris M., Coughlin, Michael, Deshmukh, Kunal P., De, Kishalay, Kasliwal, Mansi M., Morbidelli, Alessandro, Purdum, Josiah N., Quimby, Robert, Bodewits, Dennis, Chang, Chan-Kao, Ip, Wing-Huen, Hsu, Chen-Yen, Laher, Russ R., Lin, Zhong-Yi, Lisse, Carey M., Masci, Frank J., Ngeow, Chow-Choong, Tan, Hanjie, Zhai, Chengxing, Burruss, Rick, Dekany, Richard, Delacroix, Alexandre, Duev, Dmitry A., Graham, Matthew, Hale, David, Kulkarni, Shrinivas R., Kupfer, Thomas, Mahabal, Ashish, Mróz, Przemyslaw J., Neill, James D., Riddle, Reed, Rodriguez, Hector, Smith, Roger M., Soumagnac, Maayane T., Walters, Richard, Yan, Lin, Zolkower, Jeffry

We present time-resolved visible spectrophotometry of 2020 CD₃, the second known minimoon. The spectrophotometry was taken with the Keck I/Low Resolution Imaging Spectrometer between wavelengths 434 and 912 nm in the B, g, V, R, I, and RG850 filters as it was leaving the Earth–Moon system on 2020 March 23 UTC. The spectrum of 2020 CD₃ resembles V-type asteroids and some lunar rock samples with a 434–761 nm reddish slope of ~18%/100 nm (g–r = 0.62 ± 0.08 and r–i = 0.21 ± 0.06) with an absorption band at ~900 nm corresponding to i–z = −0.54 ± 0.10. Combining our measured H of 31.9 ± 0.1 with an albedo of 0.35 typical for V-type asteroids, we determine 2020 CD₃'s diameter to be ~0.9 ± 0.1 m, making it the first minimoon and one of the smallest asteroids to be spectrally studied. We use our time-series photometry to detect significant periodic light-curve variations with a period of ~573 s and amplitude of ~1. In addition, we extend the observational arc of 2020 CD₃ to 37 days, to 2020 March 23 UTC. From the improved orbital solution for 2020 CD₃, we estimate the likely duration of its capture to be ~2 yr and the nongravitational perturbation on its orbit due to radiation pressure with an area-to-mass ratio of (6.9 ± 2.4) × 10⁻⁴ m² kg⁻¹ implying a density of 2.3 ± 0.8 g cm⁻³, broadly compatible with other meter-scale asteroids and lunar rock. We searched for prediscovery detections of 2020 CD₃ in the Zwicky Transient Facility archive as far back as 2018 October but were unable to locate any positive detections.

Status
Published
Date
September 8th, 2020
Article doi
https://doi.org/10.3847/2041-8213/abae69
Discussion Paper arxiv
https://arxiv.org/abs/2008.05384
79. Helium-rich Superluminous Supernovae from the Zwicky Transient Facility
Authors
Yan, Lin, Perley, D., Schulze, S., Lunnan, R., Sollerman, J., De, K., Chen, Z., Fremling, C., Gal-Yam, A., Taggart, K., Chen, T. W., Andreoni, I., Bellm, E. C., Cunningham, V., Dekany, R., Duev, D., Fransson, C., Laher, R., Hankins, M., Ho, A. Y. Q., Jencson, J., Kaye, S., Kulkarni, S., Kasliwal, M., Golkhou, V., Graham, M., Masci, F., Miller, A., Neill, J., Ofek, E., Porter, M., Mróz, P., Reiley, D., Riddle, R., Rigault, M., Rusholme, B., Shupe, D., Soumagnac, M., Smith, R., Tartaglia, L., Yao, Y., Yaron, O.

Helium is expected to be present in the massive ejecta of some hydrogen-poor superluminous supernovae (SLSN-I). However, until now only one event has been identified with He features in its photospheric spectra (PTF10hgi). We present the discovery of a new He-rich SLSN-I, ZTF19aawfbtg (SN2019hge), at z = 0.0866. This event has more than 10 optical spectra at phases from −41 to +103 days relative to the peak, most of which match well with that of PTF10hgi. Confirmation comes from a near-IR spectrum taken at +34 days, revealing He I features with P-Cygni profiles at 1.083 and 2.058 μm. Using the optical spectra of PTF10hgi and SN2019hge as templates, we examined 70 other SLSNe-I discovered by Zwicky Transient Facility in the first two years of operation and found five additional SLSNe-I with distinct He-features. The excitation of He I atoms in normal core-collapse supernovae requires nonthermal radiation, as proposed by previous studies. These He-rich events cannot be explained by the traditional ⁵⁶Ni mixing model because of their blue spectra, high peak luminosities, and long rise timescales. Magnetar models offer a possible solution since pulsar winds naturally generate high-energy particles, potential sources of nonthermal excitation. An alternative model is the interaction between the ejecta and dense H-poor circumstellar material, which may be supported by observed undulations in the light curves. These six SLSNe-Ib have relatively low-peak luminosities (rest frame M_g = −20.06 ± 0.16).

Status
Published
Date
August 25th, 2020
Article doi
https://doi.org/10.3847/2041-8213/abb8c5
Discussion Paper arxiv
https://arxiv.org/abs/2006.13758
78. ZTF20aajnksq (AT2020blt): A Fast Optical Transient at z ≈ 2.9 With No Detected Gamma-Ray Burst Counterpart
Authors
Ho, Anna Y. Q., Perley, Daniel A., Beniamini, Paz, Cenko, S. Bradley, Kulkarni, S. R., Andreoni, Igor, Singer, Leo P., De, Kishalay, Kasliwal, Mansi M., Fremling, Christoffer, Bellm, Eric C., Dekany, Richard, Delacroix, Alexandre, Duev, Dmitry A., Goldstein, Daniel A., Golkhou, V. Zach, Goobar, Ariel, Hale, David, Kupfer, Thomas, Laher, Russ R., Masci, Frank J., Miller, A. A., Neill, James D., Riddle, Reed, Rusholme, Ben, Shupe, David L., Smith, Roger, Sollerman, Jesper, van Roestel, Jan

We present ZTF20aajnksq (AT2020blt), a fast-fading (Δr = 2.4 mag in Δt = 1.3 days) red (g−r ≈ 0.6 mag) and luminous (M₁₆₂₆ = −25.9) optical transient at z = 2.9 discovered by the Zwicky Transient Facility (ZTF). AT2020blt shares several features in common with afterglows to long-duration gamma-ray bursts (GRBs): (1) an optical light curve well-described by a broken power-law with a break at t_j = 1 day (observer-frame); (2) a luminous (L_X = 10⁴⁶ erg s⁻¹) X-ray counterpart; and (3) luminous (L_ν = 4×10³¹ erg sec⁻¹ Hz−1 at 10 GHz) radio emission. However, no GRB was detected in the 0.74d between the last ZTF non-detection (r > 20.64) and the first ZTF detection (r = 19.57), with an upper limit on the isotropic-equivalent gamma-ray energy release of E_(γ,iso) < 7×10⁵² erg. AT2020blt is thus the third afterglow-like transient discovered without a detected GRB counterpart (after PTF11agg and ZTF19abvizsw) and the second (after ZTF19abvizsw) with a redshift measurement. We conclude that the properties of AT2020blt are consistent with a classical (initial Lorentz factor Γ₀ ≳ 100) on-axis GRB that was missed by high-energy satellites. Furthermore, by estimating the rate of transients with light curves similar to that of AT2020blt in ZTF high-cadence data, we agree with previous results that there is no evidence for an afterglow-like phenomenon that is significantly more common than classical GRBs. We conclude by discussing the status and future of fast-transient searches in wide-field high-cadence optical surveys.

Status
Unpublished
Date
August 24th, 2020
Discussion Paper arxiv
https://arxiv.org/abs/2006.10761v2
77. A New Class of Roche Lobe–filling Hot Subdwarf Binaries
Authors
Kupfer, Thomas, Bauer, Evan B., Burdge, Kevin B., van Roestel, Jan, Bellm, Eric C., Fuller, Jim, Hermes, JJ, Marsh, Thomas R., Bildsten, Lars, Kulkarni, Shrinivas R., Phinney, E. S., Prince, Thomas A., Szkody, Paula, Yao, Yuhan, Irrgang, Andreas, Heber, Ulrich, Schneider, David, Dhillon, Vik S., Murawski, Gabriel, Drake, Andrew J., Duev, Dmitry A., Feeney, Michael, Graham, Matthew J., Laher, Russ R., Littlefair, S. P., Mahabal, A. A., Masci, Frank J., Porter, Michael, Reiley, Dan, Rodriguez, Hector, Rusholme, Ben, Shupe, David L., Soumagnac, Maayane T.

We present the discovery of the second binary with a Roche lobe–filling hot subdwarf transferring mass to a white dwarf (WD) companion. This 56 minute binary was discovered using data from the Zwicky Transient Facility. Spectroscopic observations reveal an He-sdOB star with an effective temperature of T_(eff) = 33,700 ± 1000 K and a surface gravity of log(g) = 5.54 ± 0.11. The GTC+HiPERCAM light curve is dominated by the ellipsoidal deformation of the He-sdOB star and shows an eclipse of the He-sdOB by an accretion disk as well as a weak eclipse of the WD. We infer a He-sdOB mass of M_(sdOB) = 0.41 ± 0.04 M_⊙ and a WD mass of M WD = 0.68 ± 0.05 M_⊙. The weak eclipses imply a WD blackbody temperature of 63,000 ± 10,000 K and a radius R_(WD) = 0.0148 ± 0.0020 R_⊙ as expected for a WD of such high temperature. The He-sdOB star is likely undergoing hydrogen shell burning and will continue transferring mass for ≈1 Myr at a rate of 10⁻⁹ M_⊙ yr⁻¹, which is consistent with the high WD temperature. The hot subdwarf will then turn into a WD and the system will merge in ≈30 Myr. We suggest that Galactic reddening could bias discoveries toward preferentially finding Roche lobe–filling systems during the short-lived shell-burning phase. Studies using reddening-corrected samples should reveal a large population of helium core–burning hot subdwarfs with T_(eff) ≈ 25,000 K in binaries of 60–90 minutes with WDs. Though not yet in contact, these binaries would eventually come into contact through gravitational-wave emission and explode as a subluminous thermonuclear supernova or evolve into a massive single WD.

Status
Published
Date
July 24th, 2020
Article doi
https://doi.org/10.3847/2041-8213/aba3c2
Discussion Paper arxiv
https://arxiv.org/abs/2007.05349
76. Optical follow-up of the neutron star–black hole mergers S200105ae and S200115j
Authors
Anand, Shreya, Coughlin, Michael W., Kasliwal, Mansi M., Bulla, Mattia, Ahumada, Tomás, Sagués Carracedo, Ana, Almualla, Mouza, Andreoni, Igor, Stein, Robert, Foucart, François, Singer, Leo P., Sollerman, Jesper, Bellm, Eric C., Bolin, Bryce, Caballero-García, M. D., Castro-Tirado, Alberto J., Cenko, S. Bradley, De, Kishalay, Dekany, Richard G., Duev, Dmitry A., Feeney, Michael, Fremling, Christoffer, Goldstein, Daniel A., Golkhou, V. Zach, Graham, Matthew J., Guessoum, Nidhal, Hankins, Matthew J., Hu, Youdong, Kong, Albert K. H., Kool, Erik C., Kulkarni, S. R., Kumar, Harsh, Laher, Russ R., Masci, Frank J., Mróz, Przemek, Nissanke, Samaya, Porter, Michael, Reusch, Simeon, Riddle, Reed, Rosnet, Philippe, Rusholme, Ben, Serabyn, Eugene, Sánchez-Ramírez, R., Rigault, Mickael, Shupe, David L., Smith, Roger, Soumagnac, Maayane T., Walters, Richard, Valeev, Azamat F.

LIGO and Virgo’s third observing run revealed the first neutron star–black hole (NSBH) merger candidates in gravitational waves. These events are predicted to synthesize r-process elements creating optical/near-infrared ‘kilonova’ emission. The joint gravitational wave and electromagnetic detection of an NSBH merger could be used to constrain the equation of state of dense nuclear matter, and independently measure the local expansion rate of the Universe. Here, we present the optical follow-up and analysis of two of the only three high-significance NSBH merger candidates detected to date, S200105ae and S200115j, with the Zwicky Transient Facility. The Zwicky Transient Facility observed ~48% of S200105ae and ~22% of S200115j’s localization probabilities, with observations sensitive to kilonovae brighter than −17.5 mag fading at 0.5 mag d⁻¹ in the g- and r-bands; extensive searches and systematic follow-up of candidates did not yield a viable counterpart. We present state-of-the-art kilonova models tailored to NSBH systems that place constraints on the ejecta properties of these NSBH mergers. We show that with observed depths of apparent magnitude ~22 mag, attainable in metre-class, wide-field-of-view survey instruments, strong constraints on ejecta mass are possible, with the potential to rule out low mass ratios, high black hole spins and large neutron star radii.

Status
In Press
Date
July 24th, 2020
Article doi
https://doi.org/10.1038/s41550-020-1183-3
Free ReadCube access pub
https://rdcu.be/b7bUU
Discussion Paper arxiv
https://arxiv.org/abs/2009.07210
75. Candidate Electromagnetic Counterpart to the Binary Black Hole Merger Gravitational-Wave Event S190521g
Authors
Graham, M. J., Ford, K. E. S., McKernan, B., Ross, N. P., Stern, D., Burdge, K., Coughlin, M., Djorgovski, S. G., Drake, A. J., Duev, D., Kasliwal, M., Mahabal, A. A., van Velzen, S., Belecki, J., Bellm, E. C., Burruss, R., Cenko, S. B., Cunningham, V., Helou, G., Kulkarni, S. R., Masci, F. J., Prince, T., Reiley, D., Rodriguez, H., Rusholme, B., Smith, R. M., Soumagnac, M. T.

We report the first plausible optical electromagnetic counterpart to a (candidate) binary black hole merger. Detected by the Zwicky Transient Facility, the electromagnetic flare is consistent with expectations for a kicked binary black hole merger in the accretion disk of an active galactic nucleus [B. McKernan, K. E. S. Ford, I. Bartos et al., Astrophys. J. Lett. 884, L50 (2019)] and is unlikely [< O (0.01%))] due to intrinsic variability of this source. The lack of color evolution implies that it is not a supernova and instead is strongly suggestive of a constant temperature shock. Other false-positive events, such as microlensing or a tidal disruption event, are ruled out or constrained to be < O(0.1%). If the flare is associated with S190521g, we find plausible values of total mass M_(BBH) ∼ 100 M_⊙, kick velocity v_k ∼ 200 km s⁻¹ at θ ∼ 60° in a disk with aspect ratio H/a ∼ 0.01 (i.e., disk height H at radius a) and gas density ρ ∼ 10⁻¹⁰ g cm⁻³. The merger could have occurred at a disk migration trap (a ∼ 700r_g; r_g ≡ GM_(SMBH)/c², where M_(SMBH) is the mass of the active galactic nucleus supermassive black hole). The combination of parameters implies a significant spin for at least one of the black holes in S190521g. The timing of our spectroscopy prevents useful constraints on broad-line asymmetry due to an off-center flare. We predict a repeat flare in this source due to a reencountering with the disk in ∼1.6 yr(M_SMBH)/10⁸ M_⊙)(a/10³r_g)^(3/2).

Status
Published
Date
June 25th, 2020
Article doi
https://doi.org/10.1103/PhysRevLett.124.251102
Physics : Focus featured_in
https://physics.aps.org/articles/v13/101
Discussion Paper arxiv
https://arxiv.org/abs/2006.14122
74. A Search for Extra-Tidal RR Lyrae in the Globular Cluster NGC 5024 and NGC 5053
Authors
Ngeow, Chow-Choong, Belecki, Justin, Burruss, Rick, Drake, Andrew J., Graham, Matthew J., Kaplan, David L., Kupfer, Thomas, Mahabal, Ashish, Masci, Frank J., Riddle, Reed, Rodriguez, Hector, Rusholme, Ben

Recently, Kundu et al (2019) reported that the globular cluster NGC 5024 (M53) possesses five extra-tidal RR Lyrae. In fact, four of them were instead known members of a nearby globular cluster NGC 5053. The status of the remaining extra-tidal RR Lyrae is controversial depending on the adopted tidal radius of NGC 5024. We have also searched for additional extra-tidal RR Lyrae within an area of ∼8~deg² covering both globular clusters. This includes other known RR Lyrae within the search area, as well as stars that fall within the expected range of magnitudes and colors for RR Lyrae (and yet outside the cutoff of 2/3 of the tidal radii of each globular clusters for something to be called "extra-tidal") if they were extra-tidal RR Lyrae candidates for NGC 5024 or NGC 5053. Based on the the proper motion information and their locations on the color-magnitude diagram, none of the known RR Lyrae belong to the extra-tidal RR Lyrae of either globular clusters. In the cases where the stars satisfied the magnitude and color ranges of RR Lyrae, analysis of time series data taken from the Zwicky Transient Facility do not reveal periodicities, suggesting that none of these stars are RR Lyrae. We conclude that there are no extra-tidal RR Lyrae associated with either NGC 5024 or NGC 5053 located within our search area.

Status
Published
Date
June 8th, 2020
Article doi
https://doi.org/10.3847/1538-3881/ab930b
Discussion Paper arxiv
https://arxiv.org/abs/2005.06088
73. The Zwicky Transient Facility Census of the Local Universe I: Systematic search for Calcium rich gap transients reveal three related spectroscopic sub-classes
Authors
De, Kishalay, Kasliwal, Mansi M., Tzanidakis, Anastasios, Fremling, U. Christoffer, Adams, Scott, Andreoni, Igor, Bagdasaryan, Ashot, Bellm, Eric C., Bildsten, Lars, Cannella, Christopher, Cook, David O., Delacroix, Alexandre, Drake, Andrew, Duev, Dmitry, Dugas, Alison, Frederick, Sara, Gal-Yam, Avishay, Goldstein, Daniel, Golkhou, V. Zach, Graham, Matthew J., Hale, David, Hankins, Matthew, Helou, George, Ho, Anna Y. Q., Irani, Ido, Jencson, Jacob E., Kaye, Stephen, Kulkarni, S. R., Kupfer, Thomas, Laher, Russ R., Leadbeater, Robin, Lunnan, Ragnhild, Masci, Frank J., Miller, Adam A., Neill, James D., Ofek, Eran O., Perley, Daniel A., Polin, Abigail, Prince, Thomas A., Quataert, Eliot, Reiley, Dan, Riddle, Reed L., Rusholme, Ben, Sharma, Yashvi, Shupe, David L., Sollerman, Jesper, Tartaglia, Leonardo, Walters, Richard, Yan, Lin, Yao, Yuhan

Using the Zwicky Transient Facility alert stream, we are conducting a large campaign to spectroscopically classify all transients occurring in galaxies in the Census of the Local Universe (CLU) catalog. The aim of the experiment is to construct a spectroscopically complete, volume-limited sample of transients coincident within 100" of CLU galaxies out to 200Mpc, and to a depth of 20 mag. We describe the survey design and spectroscopic completeness from the first 16 months of operations. We present results from a systematic search for Calcium rich gap transients in the sample of 22 low luminosity (peak absolute magnitude M > -17), hydrogen poor events found in the experiment (out of 754 spectroscopically classified SNe). We report the detection of eight Calcium rich gap transients, and place a lower limit on the volumetric rate of these events to be ≈ 15±5% of the SN Ia rate. Combining this sample with ten events from the literature, we find evidence of a likely continuum of spectroscopic properties ranging from events with SN Ia-like features (Ca-Ia objects) to SN Ib/c-like features (Ca-Ib/c objects) at peak light. Within the Ca-Ib/c events, we find two populations of events distinguished by their red (g - r ≈ 1.5 mag) or green (g - r ≈ 0.5 mag) spectral colors at r-band peak. Red Ca-Ib/c events are characterized by strongly line-blanketed spectra, systematically slower light curves, weaker and slower (by ≈ 3000 km s⁻¹) He lines and lower [Ca II]/[O I] flux ratio (by a factor of ≈ 2) in the nebular phase compared to the green Ca-Ib/c objects. Ca-Ia objects show typically more luminous light curves (M_r ≈ -16.9 mag at peak) than Ca-Ib/c objects, strong line blanketing signatures with intermediate red colors (g - r ≈ 1 mag at r-band peak), slow light curves, and weak or no [O I] in the nebular phase. Together, we find that the continuum of spectroscopic properties and the correlations thereof, the volumetric rates and striking old environments of these events are consistent with progenitor channels involving the explosive burning of He shells on low mass white dwarfs over a range of shell and core masses. We posit that the Ca-Ia and red Ca-Ib/c objects are broadly consistent with scenarios invoking the double detonation of He shells on white dwarfs with high He burning efficiency, while green Ca-Ib/c objects could arise from He shell explosions with lower He burning efficiency such as detonations in lower density He shells or He shell deflagrations.

Status
Unpublished
Date
June 5th, 2020
Discussion Paper arxiv
http://arxiv.org/abs/2004.09029
72. A high-energy neutrino coincident with a tidal disruption event
Authors
Stein, Robert, van Velzen, Sjoert, Kowalski, Marek, Franckowiak, Anna, Gezari, Suvi, Miller-Jones, James C. A., Frederick, Sara, Sfaradi, Itai, Bietenholz, Michael F., Horesh, Assaf, Fender, Rob, Garrappa, Simone, Ahumada, Tomás, Andreoni, Igor, Belicki, Justin, Bellm, Eric C., Böttcher, Markus, Brinnel, Valery, Burruss, Rick, Cenko, S. Bradley, Coughlin, Michael W., Cunningham, Virginia, Drake, Andrew, Farrar, Glennys R., Feeney, Michael, Foley, Ryan J., Gal-Yam, Avishay, Golkhou, V. Zach, Goobar, Ariel, Graham, Matthew J., Hammerstein, Erica, Helou, George, Hung, Tiara, Kasliwal, Mansi M., Kilpatrick, Charles D., Kong, Albert K. H., Kupfer, Thomas, Laher, Russ R., Mahabal, Ashish A., Masci, Frank J., Necker, Jannis, Nordin, Jakob, Perley, Daniel A., Rigault, Mickael, Reusch, Simeon, Rodriguez, Hector, Rojas-Bravo, César, Rusholme, Ben, Shupe, David L., Singer, Leo P., Sollerman, Jesper, Soumagnac, Maayane T., Stern, Daniel, Taggart, Kirsty, van Santen, Jakob, Ward, Charlotte, Woudt, Patrick A., Yao, Yuhan

Cosmic neutrinos provide a unique window into the otherwise-hidden mechanism of particle acceleration in astrophysical objects. A flux of high-energy neutrinos was discovered in 2013, and the IceCube Collaboration recently associated one high-energy neutrino with a flare from the relativistic jet of an active galaxy pointed towards the Earth. However a combined analysis of many similar active galaxies revealed no excess from the broader population, leaving the vast majority of the cosmic neutrino flux unexplained. Here we present the association of a radio-emitting tidal disruption event (AT2019dsg) with another high-energy neutrino, identified as part of our systematic search for optical counterparts to high-energy neutrinos with the Zwicky Transient Facility (ZTF). The probability of finding any radio-emitting tidal disruption event by chance is 0.5%, while the probability of finding one as bright in bolometric energy flux as AT2019dsg is 0.2%. Our electromagnetic observations can be explained through a multi-zone model, with radio analysis revealing a central engine, embedded in a UV photosphere, that powers an extended synchrotron-emitting outflow. This provides an ideal site for PeV neutrino production. The association suggests that tidal disruption events contribute to the cosmic neutrino flux. Unlike previous work which considered the rare subset of tidal disruption events with relativistic jets, our observations of AT2019dsg suggest an empirical model with a mildly-relativistic outflow.

Status
Unpublished
Date
June 5th, 2020
Discussion Paper arxiv
http://arxiv.org/abs/2005.05340
71. Zwicky Transient Facility Constraints on the Optical Emission from the Nearby Repeating FRB 180916.J0158+65
Authors
Andreoni, Igor, Lu, Wenbin, Smith, Roger M., Masci, Frank J., Bellm, Eric C., Graham, Matthew J., Kaplan, David L., Kasliwal, Mansi M., Kaye, Stephen, Kupfer, Thomas, Laher, Russ R., Mahabal, Ashish A., Nordin, Jakob, Porter, Michael, Prince, Thomas A., Reiley, Dan, Riddle, Reed, van Roestel, Joannes, Yao, Yuhan

The discovery rate of fast radio bursts (FRBs) is increasing dramatically thanks to new radio facilities. Meanwhile, wide-field instruments such as the 47 deg² Zwicky Transient Facility (ZTF) survey the optical sky to study transient and variable sources. We present serendipitous ZTF observations of the Canadian Hydrogen Intensity Mapping Experiment (CHIME) repeating source FRB 180916.J0158+65 that was localized to a spiral galaxy 149 Mpc away and is the first FRB suggesting periodic modulation in its activity. While 147 ZTF exposures corresponded to expected high-activity periods of this FRB, no single ZTF exposure was at the same time as a CHIME detection. No >3σ optical source was found at the FRB location in 683 ZTF exposures, totaling 5.69 hr of integration time. We combined ZTF upper limits and expected repetitions from FRB 180916.J0158+65 in a statistical framework using a Weibull distribution, agnostic of periodic modulation priors. The analysis yielded a constraint on the ratio between the optical and radio fluences of η ≲ 200, corresponding to an optical energy E_(opt) ≲ 3 × 10⁴⁶ erg for a fiducial 10 Jy ms FRB (90% confidence). A deeper (but less statistically robust) constraint of η ≲ 3 can be placed assuming a rate of r(>5 Jy ms)= hr⁻¹ and 1.2 ± 1.1 FRB occurring during exposures taken in high-activity windows. The constraint can be improved with shorter per-image exposures and longer integration time, or observing FRBs at higher Galactic latitudes. This work demonstrated how current surveys can statistically constrain multiwavelength counterparts to FRBs even without deliberately scheduled simultaneous radio observation.

Status
Published
Date
June 5th, 2020
Article doi
https://doi.org/10.3847/2041-8213/ab94a5
Discussion Paper arxiv
https://arxiv.org/abs/2005.06273
70. The Spectacular Ultraviolet Flash from the Peculiar Type Ia Supernova 2019yvq
Authors
Miller, A. A., Magee, M. R., Polin, A., Maguire, K., Zimmerman, E., Yao, Y., Sollerman, J., Schulze, S., Perley, D. A., Kromer, M., Dhawan, S., Bulla, M., Andreoni, I., Bellm, E. C., De, K., Dekany, R., Delacroix, A., Fremling, C., Gal-Yam, A., Goldstein, D. A., Golkhou, V. Z., Goobar, A., Graham, M. J., Irani, I., Kasliwal, M. M., Kaye, S., Kim, Y. -L., Laher, R. R., Mahabal, A. A., Masci, F. J., Nugent, P. E., Ofek, E., Phinney, E. S., Prentice, S. J., Riddle, R., Rigault, M., Rusholme, B., Schweyer, T., Shupe, D. L., Soumagnac, M. T., Terreran, G., Walters, R., Yan, L., Zolkower, J., Kulkarni, S. R.

Early observations of Type Ia supernovae (SNe Ia) provide essential clues for understanding the progenitor system that gave rise to the terminal thermonuclear explosion. We present exquisite observations of SN 2019yvq, the second observed SN Ia, after iPTF 14atg, to display an early flash of emission in the ultraviolet (UV) and optical. Our analysis finds that SN 2019yvq was unusual, even when ignoring the initial flash, in that it was moderately underluminous for an SN Ia (M_g ~ -18.5 mag at peak) yet featured very high absorption velocities (v ~ 15,000 km s⁻¹ for Si ii λ6355 at peak). We find that many of the observational features of SN 2019yvq, aside from the flash, can be explained if the explosive yield of radioactive ⁵⁶Ni is relatively low (we measure M₅₆_(Ni) = 0.31 ± 0.05 M_⊙ and it and other iron-group elements are concentrated in the innermost layers of the ejecta. To explain both the UV/optical flash and peak properties of SN 2019yvq we consider four different models: interaction between the SN ejecta and a nondegenerate companion, extended clumps of ⁵⁶Ni in the outer ejecta, a double-detonation explosion, and the violent merger of two white dwarfs. Each of these models has shortcomings when compared to the observations; it is clear additional tuning is required to better match SN 2019yvq. In closing, we predict that the nebular spectra of SN 2019yvq will feature either H or He emission, if the ejecta collided with a companion, strong [Ca ii] emission, if it was a double detonation, or narrow [O i] emission, if it was due to a violent merger.

Status
Published
Date
June 5th, 2020
Article doi
https://doi.org/10.3847/1538-4357/ab9e05
Discussion Paper arxiv
https://arxiv.org/abs/2005.05972
69. SN2019dge: a Helium-rich Ultra-Stripped Envelope Supernova
Authors
Yao, Yuhan, De, Kishalay, Kasliwal, Mansi M., Ho, Anna Y. Q., Schulze, Steve, Li, Zhihui, Kulkarni, S. R., Fruchter, Andrew, Rubin, David, Perley, Daniel A., Fuller, Jim, Piro, Anthony L., Fremling, C., Bellm, Eric C., Burruss, Rick, Duev, Dmitry A., Feeney, Michael, Gal-Yam, Avishay, Golkhou, V. Zach, Graham, Matthew J., Helou, George, Kupfer, Thomas, Laher, Russ R., Masci, Frank J., Miller, Adam A., Rusholme, Ben, Shupe, David L., Smith, Roger, Sollerman, Jesper, Soumagnac, Maayane T., Zolkower, Jeffry

We present observations of ZTF18abfcmjw (SN2019dge), a helium-rich supernova with a fast-evolving light curve indicating an extremely low ejecta mass (≈0.33 M_⊙) and low kinetic energy (≈1.3 × 10⁵⁰ erg). Early-time (<4 days after explosion) photometry reveals evidence of shock cooling from an extended helium-rich envelope of ~0.1 M_⊙ located ~1.2 × 10¹³ cm from the progenitor. Early-time He II line emission and subsequent spectra show signatures of interaction with helium-rich circumstellar material, which extends from ≳ 5 × 10¹³ cm to 2 × 10¹⁶ cm. We interpret SN2019dge as a helium-rich supernova from an ultra-stripped progenitor, which originates from a close binary system consisting of a mass-losing helium star and a low-mass main-sequence star or a compact object (i.e., a white dwarf, a neutron star, or a black hole). We infer that the local volumetric birth rate of 19dge-like ultra-stripped SNe is in the range of 1400–8200 Gpc⁻³ yr⁻¹ (i.e., 2%–12% of core-collapse supernova rate). This can be compared to the observed coalescence rate of compact neutron star binaries that are not formed by dynamical capture.

Status
Published
Date
June 5th, 2020
Article doi
https://doi.org/10.3847/1538-4357/abaa3d
Discussion Paper arxiv
https://arxiv.org/abs/2005.12922
68. SN 2020bvc: A Broad-line Type Ic Supernova with a Double-peaked Optical Light Curve and a Luminous X-Ray and Radio Counterpart
Authors
Ho, Anna Y. Q., Kulkarni, S. R., Perley, Daniel A., Cenko, S. Bradley, Corsi, Alessandra, Schulze, Steve, Lunnan, Ragnhild, Sollerman, Jesper, Gal-Yam, Avishay, Anand, Shreya, Barbarino, Cristina, Bellm, Eric C., Bruch, Rachel J., Burns, Eric, De, Kishalay, Dekany, Richard, Delacroix, Alexandre, Duev, Dmitry A., Frederiks, Dmitry D., Fremling, Christoffer, Goldstein, Daniel A., Golkhou, V. Zach, Graham, Matthew J., Hale, David, Kasliwal, Mansi M., Kupfer, Thomas, Laher, Russ R., Martikainen, Julia, Masci, Frank J., Neill, James D., Ridnaia, Anna, Rusholme, Ben, Savchenko, Volodymyr, Shupe, David L., Soumagnac, Maayane T., Strotjohann, Nora L., Svinkin, Dmitry S., Taggart, Kirsty, Tartaglia, Leonardo, Yan, Lin, Zolkower, Jeffry

We present optical, radio, and X-ray observations of SN 2020bvc (=ASASSN-20bs, ZTF 20aalxlis), a nearby (z = 0.0252; d = 114 Mpc) broad-line (BL) Type Ic supernova (SN) and the first double-peaked Ic-BL discovered without a gamma-ray burst (GRB) trigger. Our observations show that SN 2020bvc shares several properties in common with the Ic-BL SN 2006aj, which was associated with the low-luminosity gamma-ray burst (LLGRB) 060218. First, the 10 GHz radio luminosity (L_(radio) ≈ 10³⁷ erg s⁻¹)is brighter than ordinary core-collapse SNe but fainter than LLGRB SNe such as SN 1998bw (associated with LLGRB 980425). We model our VLA observations (spanning 13–43 days) as synchrotron emission from a mildly relativistic (v ≳ 0.3c) forward shock. Second, with Swift and Chandra, we detect X-ray emission (L_X ≈ 10⁴¹ erg s⁻¹) that is not naturally explained as inverse Compton emission or part of the same synchrotron spectrum as the radio emission. Third, high-cadence (6× night⁻¹) data from the Zwicky Transient Facility (ZTF) show a double-peaked optical light curve, the first peak from shock cooling of extended low-mass material (mass M_e < 10⁻² M_⊙ at radius R_e > 10¹² cm) and the second peak from the radioactive decay of ⁵⁶Ni. SN 2020bvc is the first double-peaked Ic-BL SN discovered without a GRB trigger, so it is noteworthy that it shows X-ray and radio emission similar to LLGRB SNe. For four of the five other nearby (z ≾ 0.05) Ic-BL SNe with ZTF high-cadence data, we rule out a first peak like that seen in SN 2006aj and SN 2020bvc, i.e., that lasts ≈1 day and reaches a peak luminosity M ≈ −18. Follow-up X-ray and radio observations of Ic-BL SNe with well-sampled early optical light curves will establish whether double-peaked optical light curves are indeed predictive of LLGRB-like X-ray and radio emission.

Status
Published
Date
June 5th, 2020
Article doi
https://doi.org/10.3847/1538-4357/aba630
Discussion Paper arxiv
http://arxiv.org/abs/2004.10406
67. Recurring Outbursts of P/2019 LM₄ (Palomar)
Authors
Ye, Quanzhi, Kelley, Michael S. P., Bodewits, Dennis, Bauer, James M., Mahabal, Ashish, Masci, Frank J., Ngeow, Chow-Choong

We present a preliminary analysis of comet P/2019 LM₄ (Palomar) as observed by the Zwicky Transient Facility (ZTF) survey in 2019 and 2020. We find that the discovery of the comet in 2019 and the recovery in 2020 is largely attributed to two separate outbursts that are ≳2 and ≳3.9 mag in strength. The two outbursts occurred between the end of April and early May in 2019, and between 2020 May 8.31 and 9.52 UTC.

Status
Published
Date
May 29th, 2020
Article doi
https://doi.org/10.3847/2515-5172/ab96b9
66. ZTF J1901+5309: a 40.6-min orbital period eclipsing double white dwarf system
Authors
Coughlin, Michael W., Burdge, Kevin, Phinney, E. Sterl, van Roestel, Jan, Bellm, Eric C., Dekany, Richard G., Delacroix, Alexandre, Duev, Dmitry A., Feeney, Michael, Graham, Matthew J., Kulkarni, S. R., Kupfer, Thomas, Laher, Russ R., Masci, Frank J., Prince, Thomas A., Riddle, Reed, Rosnet, Philippe, Smith, Roger, Serabyn, Eugene, Walters, Richard

The Zwicky Transient Facility has begun to discover binary systems with orbital periods that are less than 1 h. Combined with dedicated follow-up systems, which allow for high-cadence photometry of these sources, systematic confirmation and characterization of these sources are now possible. Here, we report the discovery of ZTF J190125.42+530929.5, a 40.6-min orbital period, eclipsing double white dwarf binary. Both photometric modelling and spectroscopic modelling confirm its nature, yielding an estimated inclination of i=86.2^(+0.6)_(−0.2) deg and primary and secondary effective temperatures of {T}_(eff)=28000⁺⁵⁰⁰₋₅₀₀ and 17600⁺⁴⁰⁰₋₄₀₀, respectively. This system adds to a growing list of sources for future gravitational-wave detectors and contributes to the demographic analysis of double degenerates.

Status
Published
Date
April 15th, 2020
Article doi
https://doi.org/10.1093/mnrasl/slaa044
Discussion Paper arxiv
https://arxiv.org/abs/2004.00456
65. Seventeen Tidal Disruption Events from the First Half of ZTF Survey Observations: Entering a New Era of Population Studies
Authors
van Velzen, Sjoert, Gezari, Suvi, Hammerstein, Erica, Roth, Nathaniel, Frederick, Sara, Ward, Charlotte, Hung, Tiara, Cenko, S. Bradley, Stein, Robert, Perley, Daniel A., Taggart, Kirsty, Sollerman, Jesper, Andreoni, Igor, Bellm, Eric C., Brinnel, Valery, De, Kishalay, Dekany, Richard, Feeney, Michael, Foley, Ryan J., Fremling, Christoffer, Giomi, Matteo, Golkhou, V. Zach, Ho, Anna Y. Q., Kasliwal, Mansi M., Kilpatrick, Charles D., Kulkarni, Shrinivas R., Kupfer, Thomas, Laher, Russ R., Mahabal, Ashish, Masci, Frank J., Nordin, Jakob, Riddle, Reed, Rusholme, Ben, Sharma, Yashvi, van Santen, Jakob, Shupe, David L., Soumagnac, Maayane T.

While tidal disruption events (TDEs) have long been heralded as laboratories for the study of quiescent black holes, the small number of known TDEs and uncertainties in their emission mechanism have hindered progress towards this promise. Here present 17 new TDEs that have been detected recently by the Zwicky Transient Facility along with Swift UV and X-ray follow-up observations. Our homogeneous analysis of the optical/UV light curves, including 22 previously known TDEs from the literature, reveals a clean separation of light curve properties with spectroscopic class. The TDEs with Bowen fluorescence features in their optical spectra have smaller blackbody radii, as well as longer rise times and higher disruption rates compared to the rest of the sample. The Bowen fluorescence mechanism requires a high density which can be reached at smaller radii, which in turn yields longer diffusion timescales. Thus, the difference in rise times suggests the pre-peak TDE light curves are governed not by the fallback timescale, but instead by the diffusion of photons through the tidal debris. The small subset of TDEs that show only helium emission lines in their spectra have the longest rise times, the highest luminosities and the lowest rates. We also report, for the first time, the detection of soft X-ray flares from a TDE on day timescales. Based on the fact the flares peak at a luminosity similar to the optical/UV blackbody luminosity, we attribute them to brief glimpses through a reprocessing layer that otherwise obscures the inner accretion flow.

Status
Unpublished
Date
April 14th, 2020
Discussion Paper arxiv
http://arxiv.org/abs/2001.01409
64. The Broad-lined Ic Supernova ZTF18aaqjovh (SN 2018bvw): An Optically-discovered Engine-driven Supernova Candidate with Luminous Radio Emission
Authors
Ho, Anna Y. Q., Corsi, Alessandra, Cenko, S. Bradley, Taddia, Francesco, Kulkarni, S. R., Adams, Scott, De, Kishalay, Dekany, Richard, Frederiks, Dmitry D., Fremling, Christoffer, Golkhou, V. Zach, Graham, Matthew J., Hung, Tiara, Kupfer, Thomas, Laher, Russ R., Mahabal, Ashish, Masci, Frank J., Miller, Adam A., Neill, James D., Reiley, Daniel, Riddle, Reed, Ridnaia, Anna, Rusholme, Ben, Sharma, Yashvi, Sollerman, Jesper, Soumagnac, Maayane T., Svinkin, Dmitry S., Shupe, David L.

We present ZTF18aaqjovh (SN 2018bvw), a high-velocity ("broad-lined") stripped-envelope (Type Ic) supernova (Ic-BL SN) discovered in the Zwicky Transient Facility one-day cadence survey. ZTF18aaqjovh shares a number of features in common with engine-driven explosions: the photospheric velocity and the shape of the optical light curve are very similar to those of the Type Ic-BL SN 1998bw, which was associated with a low-luminosity gamma-ray burst (LLGRB) and had relativistic ejecta. However, the radio luminosity of ZTF18aaqjovh is almost two orders of magnitude fainter than that of SN 1998bw at the same velocity phase, and the shock velocity is at most mildly relativistic (v = 0.06–0.4c). A search of high-energy catalogs reveals no compelling gamma-ray burst (GRB) counterpart to ZTF18aaqjovh, and the limit on the prompt GRB luminosity of L_(γ,iso)≈1.6×10⁴⁸ erg/s⁻¹ excludes a classical GRB but not an LLGRB. Altogether, ZTF18aaqjovh represents another transition event between engine-driven SNe associated with GRBs and "ordinary" Ic-BL SNe.

63. ZTF Early Observations of Type Ia Supernovae. III. Early-time Colors As a Test for Explosion Models and Multiple Populations
Authors
Bulla, Mattia, Miller, Adam A., Yao, Yuhan, Dessart, Luc, Dhawan, Suhail, Papadogiannakis, Semeli, Biswas, Rahul, Goobar, Ariel, Kulkarni, S. R., Nordin, Jakob, Nugent, Peter, Polin, Abigail, Sollerman, Jesper, Bellm, Eric C., Coughlin, Michael W., Dekany, Richard, Golkhou, V. Zach, Graham, Matthew J., Kasliwal, Mansi M., Kupfer, Thomas, Laher, Russ R., Masci, Frank J., Porter, Michael, Rusholme, Ben, Shupe, David L.

Colors of Type Ia supernovae (SNe Ia) in the first few days after explosion provide a potential discriminant between different models. In this paper, we present g − r colors of 65 SNe Ia discovered within 5 days from first light by the Zwicky Transient Facility in 2018, a sample that is about three times larger than that in the literature. We find that g − r colors are intrinsically rather homogeneous at early phases, with about half of the dispersion attributable to photometric uncertainties (σ_(noise)∼σ_(int) ~ 0.18 mag). Colors are nearly constant starting from 6 days after first light (g − r ~ −0.15 mag), while the time evolution at earlier epochs is characterized by a continuous range of slopes, from events rapidly transitioning from redder to bluer colors (slope of ~−0.25 mag day⁻¹) to events with a flatter evolution. The continuum in the slope distribution is in good agreement both with models requiring some amount of ⁵⁶Ni mixed in the outermost regions of the ejecta and with "double-detonation" models having thin helium layers (M_(He) = 0.01 M_⊙) and varying carbon–oxygen core masses. At the same time, six events show evidence for a distinctive "red bump" signature predicted by double-detonation models with larger helium masses. We finally identify a significant correlation between the early-time g − r slopes and supernova brightness, with brighter events associated to flatter color evolution (p-value = 0.006). The distribution of slopes, however, is consistent with being drawn from a single population, with no evidence for two components as claimed in the literature based on B − V colors.

Status
Published
Date
April 14th, 2020
Article doi
https://doi.org/10.3847/1538-4357/abb13c
Discussion Paper arxiv
https://arxiv.org/abs/2001.00587
62. ZTF Early Observations of Type Ia Supernovae. II. First Light, the Initial Rise, and Time to Reach Maximum Brightness
Authors
Miller, A. A., Yao, Y., Bulla, M., Pankow, C., Bellm, E. C., Cenko, S. B., Dekany, R., Fremling, C., Graham, M. J., Kupfer, T., Laher, R. R., Mahabal, A. A., Masci, F. J., Nugent, P. E., Riddle, R., Rusholme, B., Smith, R. M., Shupe, D. L., van Roestel, J., Kulkarni, S. R.

While it is clear that Type Ia supernovae (SNe) are the result of thermonuclear explosions in C/O white dwarfs (WDs), a great deal remains uncertain about the binary companion that facilitates the explosive disruption of the WD. Here, we present a comprehensive analysis of a large, unique data set of 127 SNe Ia with exquisite coverage by the Zwicky Transient Facility (ZTF). High-cadence (six observations per night) ZTF observations allow us to measure the SN rise time and examine its initial evolution. We develop a Bayesian framework to model the early rise as a power law in time, which enables the inclusion of priors in our model. For a volume-limited subset of normal SNe Ia, we find that the mean power-law index is consistent with 2 in the r_(ZTF)-band (α_r=2.01±0.02), as expected in the expanding fireball model. There are, however, individual SNe that are clearly inconsistent with α_r = 2. We estimate a mean rise time of 18.9 days (with a range extending from ~15 to 22 days), though this is subject to the adopted prior. We identify an important, previously unknown, bias whereby the rise times for higher-redshift SNe within a flux-limited survey are systematically underestimated. This effect can be partially alleviated if the power-law index is fixed to α = 2, in which case we estimate a mean rise time of 21.7 days (with a range from ~18 to 23 days). The sample includes a handful of rare and peculiar SNe Ia. Finally, we conclude with a discussion of lessons learned from the ZTF sample that can eventually be applied to observations from the Vera C. Rubin Observatory.

Status
Published
Date
April 13th, 2020
Article doi
https://doi.org/10.3847/1538-4357/abb13b
Discussion Paper arxiv
https://arxiv.org/abs/2001.00598
61. Cataclysmic Variables in the First Year of the Zwicky Transient Facility
Authors
Szkody, Paula, Dicenzo, Brooke, Ho, Anna Y. Q., Hillenbrand, Lynne A., van Roestel, Jan, Ridder, Margaret, Lima, Isabel DeJesus, Graham, Melissa L., Bellm, Eric C., Burdge, Kevin, Kupfer, Thomas, Prince, Thomas A., Masci, Frank J., Mróz, Przemyslaw J., Golkhou, V. Zach, Coughlin, Michael, Cunningham, Virginia A., Dekany, Richard, Graham, Matthew J., Hale, David, Kaplan, David, Kasliwal, Mansi M., Miller, Adam A., Neill, James D., Patterson, Maria T., Riddle, Reed, Smith, Roger, Soumagnac, Maayanne T.

Using selection criteria based on amplitude, time, and color, we have identified 329 objects as known or candidate cataclysmic variables (CVs) during the first year of testing and operation of the Zwicky Transient Facility. Of these, 90 are previously confirmed CVs, 218 are strong candidates based on the shape and color of their light curves obtained during 3–562 days of observation, and the remaining 21 are possible CVs but with too few data points to be listed as good candidates. Almost half of the strong candidates are within 10 deg of the galactic plane, in contrast to most other large surveys that have avoided crowded fields. The available Gaia parallaxes are consistent with sampling the low mass transfer CVs, as predicted by population models. Our follow-up spectra have confirmed Balmer/helium emission lines in 27 objects, with four showing high-excitation He ii emission, including candidates for an AM CVn, a polar, and an intermediate polar. Our results demonstrate that a complete survey of the Galactic plane is needed to accomplish an accurate determination of the number of CVs existing in the Milky Way.

Status
Published
Date
March 11th, 2020
Article doi
https://doi.org/10.3847/1538-3881/ab7cce
Discussion Paper arxiv
https://arxiv.org/abs/2002.08447
60. Early Ultraviolet Observations of Type IIn Supernovae Constrain the Asphericity of Their Circumstellar Material
Authors
Soumagnac, Maayane T., Ofek, Eran O., Liang, Jingyi, Gal-Yam, Avishay, Nugent, Peter, Yang, Yi, Cenko, S. Bradley, Sollerman, Jesper, Perley, Daniel A., Andreoni, Igor, Barbarino, Cristina, Burdge, Kevin B., Bruch, Rachel J., De, Kishalay, Dugas, Alison, Fremling, Christoffer, Graham, Melissa L., Hankins, Matthew J., Strotjohann, Nora Linn, Moran, Shane, Neill, James D., Schulze, Steve, Shupe, David L., Sipőcz, Brigitta M., Taggart, Kirsty, Tartaglia, Leonardo, Walters, Richard, Yan, Lin, Yao, Yuhan, Yaron, Ofer, Bellm, Eric C., Cannella, Chris, Dekany, Richard, Duev, Dmitry A., Feeney, Michael, Frederick, Sara, Graham, Matthew J., Laher, Russ R., Masci, Frank J., Kasliwal, Mansi M., Kowalski, Marek, Miller, Adam A., Rigault, Mickael, Rusholme, Ben

We present a survey of the early evolution of 12 Type IIn supernovae (SNe IIn) at ultraviolet and visible light wavelengths. We use this survey to constrain the geometry of the circumstellar material (CSM) surrounding SN IIn explosions, which may shed light on their progenitor diversity. In order to distinguish between aspherical and spherical CSM, we estimate the blackbody radius temporal evolution of the SNe IIn of our sample, following the method introduced by Soumagnac et al. We find that higher-luminosity objects tend to show evidence for aspherical CSM. Depending on whether this correlation is due to physical reasons or to some selection bias, we derive a lower limit between 35% and 66% for the fraction of SNe IIn showing evidence for aspherical CSM. This result suggests that asphericity of the CSM surrounding SNe IIn is common—consistent with data from resolved images of stars undergoing considerable mass loss. It should be taken into account for more realistic modeling of these events.

Status
Published
Date
March 10th, 2020
Article doi
https://doi.org/10.3847/1538-4357/ab94be
Discussion Paper arxiv
https://arxiv.org/abs/2001.05518
59. The Koala: A Fast Blue Optical Transient with Luminous Radio Emission from a Starburst Dwarf Galaxy at z = 0.27
Authors
Ho, Anna Y. Q., Perley, Daniel A., Kulkarni, S. R., Dong, Dillon Z. J., De, Kishalay, Chandra, Poonam, Andreoni, Igor, Bellm, Eric C., Burdge, Kevin B., Coughlin, Michael, Dekany, Richard, Feeney, Michael, Fredericks, Dmitry D., Fremling, Christoffer, Golkhou, V. Zach, Graham, Matthew J., Hale, David, Helou, George, Horesh, Assaf, Kasliwal, Mansi M., Laher, Russ R., Masci, Frank J., Miller, A. A., Porter, Michael, Ridnaia, Anna, Rusholme, Ben, Shupe, David L., Soumagnac, Maayane T., Svinkin, Dmitry S.

We present ZTF18abvkwla (the "Koala"), a fast blue optical transient discovered in the Zwicky Transient Facility (ZTF) One-Day Cadence (1DC) Survey. ZTF18abvkwla has a number of features in common with the groundbreaking transient AT 2018cow: blue colors at peak (g – r ≈ −0.5 mag), a short rise time from half-max of under two days, a decay time to half-max of only three days, a high optical luminosity (M_(g,peak) ≈ −20.6 mag), a hot (≳40,000 K) featureless spectrum at peak light, and a luminous radio counterpart. At late times (Δt > 80 days), the radio luminosity of ZTF18abvkwla (ν_(Lν) ≳ 10⁴⁰ erg⁻¹ at 10 GHz, observer-frame) is most similar to that of long-duration gamma-ray bursts (GRBs). The host galaxy is a dwarf starburst galaxy (M ≈ 5×10⁸ M⊙, SFR ≈ 7 M⊙ yr⁻¹) that is moderately metal-enriched (log[O/H] ≈ 8.5), similar to the hosts of GRBs and superluminous supernovae. As in AT2018cow, the radio and optical emission in ZTF18abvkwla likely arise from two separate components: the radio from fast-moving ejecta (Γβc > 0.38c) and the optical from shock-interaction with confined dense material (<0.07 M⊙ in ∼10¹⁵ cm). Compiling transients in the literature with t_(rise) < 5 days and M_(peak) < −20 mag, we find that a significant number are engine-powered, and suggest that the high peak optical luminosity is directly related to the presence of this engine. From 18 months of the 1DC survey, we find that transients in this rise-luminosity phase space are at least two to three orders of magnitude less common than CC SNe. Finally, we discuss strategies for identifying such events with future facilities like the Large Synoptic Survey Telescope, as well as prospects for detecting accompanying X-ray and radio emission.

Status
Published
Date
March 9th, 2020
Article doi
https://doi.org/10.3847/1538-4357/ab8bcf
Discussion Paper arxiv
https://arxiv.org/abs/2003.01222
58. First Ultracompact Roche Lobe–Filling Hot Subdwarf Binary
Authors
Kupfer, Thomas, Bauer, Evan B., Marsh, Thomas R., van Roestel, Jan, Bellm, Eric C., Burdge, Kevin B., Coughlin, Michael W., Fuller, Jim, Hermes, JJ, Bildsten, Lars, Kulkarni, Shrinivas R., Prince, Thomas A., Szkody, Paula, Dhillon, Vik S., Murawski, Gabriel, Burruss, Rick, Dekany, Richard, Delacroix, Alex, Drake, Andrew J., Duev, Dmitry A., Feeney, Michael, Graham, Matthew J., Kaplan, David L., Laher, Russ R., Littlefair, S. P., Masci, Frank J., Riddle, Reed, Rusholme, Ben, Serabyn, Eugene, Smith, Roger M., Shupe, David L., Soumagnac, Maayane T.

We report the discovery of the first short-period binary in which a hot subdwarf star (sdOB) filled its Roche lobe and started mass transfer to its companion. The object was discovered as part of a dedicated high-cadence survey of the Galactic plane named the Zwicky Transient Facility and exhibits a period of P = 39.3401(1) minutes, making it the most compact hot subdwarf binary currently known. Spectroscopic observations are consistent with an intermediate He-sdOB star with an effective temperature of T_(eff) = 42,400 ± 300 K and a surface gravity of log(g) = 5.77 ± 0.05. A high signal-to-noise ratio GTC+HiPERCAM light curve is dominated by the ellipsoidal deformation of the sdOB star and an eclipse of the sdOB by an accretion disk. We infer a low-mass hot subdwarf donor with a mass M_(sdOB) = 0.337 ± 0.015 M⊙ and a white dwarf accretor with a mass M_(WD) = 0.545 ± 0.020 M⊙. Theoretical binary modeling indicates the hot subdwarf formed during a common envelope phase when a 2.5–2.8 M⊙ star lost its envelope when crossing the Hertzsprung gap. To match its current P_(orb), T_(eff), log(g), and masses, we estimate a post–common envelope period of P_(orb) ≈ 150 minutes and find that the sdOB star is currently undergoing hydrogen shell burning. We estimate that the hot subdwarf will become a white dwarf with a thick helium layer of ≈0.1 M⊙, merge with its carbon/oxygen white dwarf companion after ≈17 Myr, and presumably explode as a thermonuclear supernova or form an R CrB star.

Status
Published
Date
March 4th, 2020
Article doi
https://doi.org/10.3847/1538-4357/ab72ff
Discussion Paper arxiv
https://arxiv.org/abs/2002.01485
57. Gravitational Microlensing Events from the First Year of the Northern Galactic Plane Survey by the Zwicky Transient Facility
Authors
Mróz, Przemek, Street, R. A., Bachelet, E., Ofek, E. O., Bellm, E. C., Dekany, R., Duev, D. A., Gal-Yam, A., Graham, M. J., Masci, F. J., Porter, M., Rusholme, B., Smith, R. M., Soumagnac, M. T., Zolkower, J.

The Zwicky Transient Facility (ZTF) (Bellm et al. 2019; Graham et al. 2019; Masci et al. 2019) is currently surveying the entire northern sky, including dense Galactic plane fields. Here, we present preliminary results of the search for gravitational microlensing events in the ZTF data collected from the beginning of the survey (2018 March 20) through 2019 June 30.

Status
Published
Date
January 31st, 2020
Article doi
https://doi.org/10.3847/2515-5172/ab7021
Discussion Paper arxiv
https://arxiv.org/abs/2001.09584
56. A Twilight Search for Atiras, Vatiras, and Co-orbital Asteroids: Preliminary Results
Authors
Ye, Quanzhi, Masci, Frank J., Ip, Wing-Huen, Prince, Thomas A., Helou, George, Farnocchia, Davide, Bellm, Eric C., Dekany, Richard, Graham, Matthew J., Kulkarni, Shrinivas R., Kupfer, Thomas, Mahabal, Ashish, Ngeow, Chow-Choong, Reiley, Daniel J., Soumagnac, Maayane T.

Near-Earth objects (NEOs) that orbit the Sun on or within Earth's orbit are tricky to detect for Earth-based observers due to their proximity to the Sun in the sky. These small bodies hold clues to the dynamical history of the inner solar system as well as the physical evolution of planetesimals in extreme environments. Populations in this region include the Atira and Vatira asteroids, as well as Venus and Earth co-orbital asteroids. Here we present a twilight search for these small bodies, conducted using the 1.2 m Oschin Schmidt and the Zwicky Transient Facility (ZTF) camera at Palomar Observatory. The ZTF twilight survey operates at solar elongations down to 35° with a limiting magnitude of r = 19.5. During a total of 40 evening sessions and 62 morning sessions conducted between 2018 November 15 and 2019 June 23, we detected six Atiras, including two new discoveries, 2019 AQ₃ and 2019 LF₆, but no Vatiras or Earth/Venus co-orbital asteroids. NEO population models show that these new discoveries are likely only the tip of the iceberg, with the bulk of the population yet to be found. The population models also suggest that we have only detected 5%–7% of the H < 20 Atira population over the seven month survey. Co-orbital asteroids are smaller in diameter and require deeper surveys. A systematic and efficient survey of the near-Sun region will require deeper searches and/or facilities that can operate at small solar elongations.

Status
Published
Date
January 25th, 2020
Article doi
https://doi.org/10.3847/1538-3881/ab629c
Discussion Paper arxiv
https://arxiv.org/abs/1912.06109
55. MPEC 2020-A99: 2020 AV2
Authors
Bolin, B. T., Masci, F. J., Ye, Q.-Z.

[no abstract]

Status
Published
Date
January 15th, 2020
Article pub
https://minorplanetcenter.net/mpec/K20/K20A99.html
54. The Zwicky Transient Facility: Observing System
Authors
Dekany, Richard, Smith, Roger M., Riddle, Reed, Feeney, Michael, Porter, Michael, Hale, David, Zolkower, Jeffry, Belicki, Justin, Kaye, Stephen, Henning, John, Walters, Richard, Cromer, John, Delacroix, Alex, Rodriguez, Hector, Reiley, Daniel J., Mao, Peter, Hover, David, Murphy, Patrick, Burruss, Rick, Baker, John, Kowalski, Marek, Reif, Klaus, Mueller, Phillip, Bellm, Eric, Graham, Matthew, Kulkarni, Shrinivas R.

The Zwicky Transient Facility (ZTF) Observing System (OS) is the data collector for the ZTF project to study astrophysical phenomena in the time domain. ZTF OS is based upon the 48 inch aperture Schmidt-type design Samuel Oschin Telescope at the Palomar Observatory in Southern California. It incorporates new telescope aspheric corrector optics, dome and telescope drives, a large-format exposure shutter, a flat-field illumination system, a robotic bandpass filter exchanger, and the key element: a new 47-square-degree, 600 megapixel cryogenic CCD mosaic science camera, along with supporting equipment. The OS collects and delivers digitized survey data to the ZTF Data System (DS). Here, we describe the ZTF OS design, optical implementation, delivered image quality, detector performance, and robotic survey efficiency.

Status
Published
Date
December 7th, 2019
Article doi
https://doi.org/10.1088/1538-3873/ab4ca2
Discussion Paper arxiv
https://arxiv.org/abs/2008.04923
53. The ZTF Alert Stream: Lessons from the First Six Months of Operating an LSST Precursor
Authors
Jurić, Mario, Bellm, Eric C., Patterson, Maria T., Golkhou, V. Zach, Rusholme, Benjamin

The Zwicky Transient Facility (ZTF; Bellm et al. 2019) is an optical time-domain survey that is currently generating about one million alerts each night for transient, variable, and moving objects. The ZTF Alert Distribution System(ZADS; Patterson et al. 2019) packages these alerts, distributes them to the ZTF Partnership members and community brokers, and allows for filtering of the alerts to objects of interest, all in near-real time. This system builds on industry-standard real-time stream processing tools: the Apache Avro binary serialization format and the Apache Kafka distributed streaming platform. ZADS routinely transports 0.6 to 1.2 million alerts per night (amounting to 70GB/night), and has handled peaks of over 2 million alerts/night with no technical issues.

Status
Published
Date
December 2nd, 2019
Article pub
http://aspbooks.org/a/volumes/article_details/?paper_id=39290
52. The luminous and rapidly evolving SN 2018bcc: Clues toward the origin of Type Ibn SNe from the Zwicky Transient Facility
Authors
Karamehmetoglu, E., Fransson, C., Sollerman, J., Tartaglia, L., Taddia, F., De, K., Fremling, C., Bagdasaryan, A., Barbarino, C., Bellm, E. C., Dekaney, R., Dugas, A. M., Giomi, M., Goobar, A., Graham, M., Ho, A., Laher, R. R., Masci, F. J., Neill, J. D., Perley, D., Riddle, R., Rusholme, B., Soumagnac, M. T.

Context: Type Ibn supernovae (SNe) are rapidly-evolving and luminous (M_(R,peak) ∼−19) transients interacting with He-rich circumstellar material (CSM). SN 2018bcc, detected by the ZTF shortly after explosion, provides the best constraints on the shape of the rising lightcurve of a fast Type Ibn. Aims: We use the high-quality data set of SN 2018bcc to study observational signatures of the class. Additionally, the powering mechanism of SN 2018bcc offers insights into the debated progenitor connection of Type Ibn SNe. Methods: Well-constrained lightcurve properties obtained from empirical models are compared with the literature. We fit the pseudo-bolometric lightcurve with semi-analytical models powered by radioactive decay and CSM interaction. The line profiles and emissivity of the prominent He I lines are modeled to study the formation of P-Cygni profiles and to estimate CSM properties. Results: SN 2018bcc has a risetime to peak of 5.6^(+0.2)_(−0.1) days in the rest frame with a rising shape powerlaw index near 2, and seems to be a typical rapidly-evolving Type Ibn SN. The spectrum lacks signatures of SN-like ejecta and is dominated by over 15 He emission features at 20 days past peak, alongside Ca and Mg, all with V_(FWHM) ∼ 2000 km s⁻¹. The luminous and rapidly-evolving lightcurve can be powered by CSM interaction but not by the decay of radioactive ⁵⁶Ni. Modeling of the He I lines indicates a dense and optically thick CSM that can explain the P-Cygni profiles. Conclusions: Like other rapidly-evolving Type Ibn SNe, SN 2018bcc is a luminous transient with a rapid rise to peak powered by shock-interaction inside a dense and He-rich CSM. Its spectra do not support the existence of two Type Ibn spectral classes. We also note the remarkable observational match to pulsational pair instability (PPI) SN models.

Status
Unpublished
Date
November 21st, 2019
Discussion Paper arxiv
http://arxiv.org/abs/1910.06016
51. ZTF Early Observations of Type Ia Supernovae I: Properties of the 2018 Sample
Authors
Yao, Yuhan, Miller, Adam A., Kulkarni, S. R., Bulla, Mattia, Masci, Frank J., Goldstein, Daniel A., Goobar, Ariel, Nugent, Peter, Dugas, Alison, Blagorodnova, Nadia, Neill, James D., Rigault, Michael, Sollerman, Jesper, Nordin, J., Bellm, Eric C., Cenko, S. Bradley, De, Kishalay, Dhawan, Suhail, Feindt, Ulrich, Fremling, C., Gatkine, Pradip, Graham, Matthew J., Graham, Melissa L., Ho, Anna Y. Q., Hung, T., Kasliwal, Mansi M., Kupfer, Thomas, Laher, Russ R., Perley, Daniel A., Rusholme, Ben, Shupe, David L., Soumagnac, Maayane T., Taggart, K., Walters, Richard, Yan, Lin

Early-time observations of Type Ia supernovae (SNe Ia) are essential to constrain the properties of their progenitors. In this paper, we present high-quality light curves of 127 SNe Ia discovered by the Zwicky Transient Facility (ZTF) in 2018. We describe our method to perform forced point-spread function photometry, which can be applied to other types of extragalactic transients. With a planned cadence of six observations per night (three g + three r), all of the 127 SNe Ia are detected in both g and r bands more than 10 days (in the rest frame) prior to the epoch of g-band maximum light. The redshifts of these objects range from z = 0.0181 to 0.165; the median redshift is 0.074. Among the 127 SNe, 50 are detected at least 14 days prior to maximum light (in the rest frame), with a subset of nine objects being detected more than 17 days before g-band peak. This is the largest sample of young SNe Ia collected to date; it can be used to study the shape and color evolution of the rising light curves in unprecedented detail. We discuss six peculiar events in this sample: one 02cx-like event ZTF18abclfee (SN 2018crl), one Ia-CSM SN ZTF18aaykjei (SN 2018cxk), and four objects with possible super-Chandrasekhar mass progenitors: ZTF18abhpgje (SN 2018eul), ZTF18abdpvnd (SN 2018dvf), ZTF18aawpcel (SN 2018cir), and ZTF18abddmrf (SN 2018dsx).

Status
Published
Date
November 21st, 2019
Article doi
https://doi.org/10.3847/1538-4357/ab4cf5
Discussion Paper arxiv
https://arxiv.org/abs/1910.02967
50. Pre-discovery Activity of New Interstellar Comet 2I/Borisov Beyond 5 AU
Authors
Ye, Quanzhi, Kelley, Michael S. P., Bolin, Bryce T., Bodewits, Dennis, Farnocchia, Davide, Masci, Frank J., Meech, Karen J., Micheli, Marco, Weryk, Robert, Bellm, Eric C., Christensen, Eric, Dekany, Richard, Delacroix, Alexandre, Graham, Matthew J., Kulkarni, Shrinivas R., Laher, Russ R., Rusholme, Ben, Smith, Roger M.

Comet 2I/Borisov, the first unambiguous interstellar comet ever found, was discovered in 2019 August at ~3 au from the Sun on its inbound leg. No pre-discovery detection beyond 3 au has yet been reported, mostly due to the comet's proximity to the Sun as seen from the Earth. Here we present a search for pre-discovery detections of comet Borisov using images taken by the Catalina Sky Survey, Pan-STARRS, and the Zwicky Transient Facility (ZTF), with a further comprehensive follow-up campaign being presented in Bolin et al. We identified comet Borisov in ZTF images taken in 2019 May and use these data to update its orbit. This allowed us to identify the comet in images acquired as far back as 2018 December, when it was 7.8 au from the Sun. The comet was not detected in 2018 November when it was 8.6 au from the Sun, possibly implying an onset of activity around this time. This suggests that the activity of the comet is either driven by a more volatile species other than H₂O, such as CO or CO₂, or by exothermic crystallization of amorphous ice. We derive the radius of the nucleus to be <7 km using the non-detection in 2018 November, and estimate an area of ~0.5–10 km² has been active between 2018 December and 2019 September, though this number is model-dependent and is highly uncertain. The behavior of comet Borisov during its inbound leg is observationally consistent with dynamically new comets observed in our solar system, suggesting some similarities between the two.

Status
Published
Date
November 21st, 2019
Article doi
https://doi.org/10.3847/1538-3881/ab659b
Discussion Paper arxiv
https://arxiv.org/abs/1911.05902
49. Four (Super)luminous Supernovae from the First Months of the ZTF Survey
Authors
Lunnan, R., Yan, Lin, Perley, D. A., Schultze, S., Taggart, K., Gal-Yam, A., Fremling, C., Soumagnac, M. T., Ofek, E. O., Adams, S. M., Barbarino, C., Bellm, E. C., De, K., Fransson, C., Frederick, S., Golkhou, V. Z., Graham, M. J., Hallakoun, N., Ho, A. Y. Q., Kasliwal, M. M., Kaspi, S., Kulkarni, S. R., Laher, R. R., Masci, F. J., Nuñez, F. Pozo, Rusholme, B., Quimby, R. M., Shupe, D. L., Sollerman, J., Taddia, F., van Roestel, J., Yang, Y., Yao, Yuhan

We present photometry and spectroscopy of four hydrogen-poor luminous supernovae discovered during the 2-month long science commissioning and early operations of the Zwicky Transient Facility (ZTF) survey. Three of these objects, SN 2018bym (ZTF18aapgrxo), SN 2018avk (ZTF18aaisyyp), and SN 2018bgv (ZTF18aavrmcg), resemble typical SLSN-I spectroscopically, while SN 2018don (ZTF18aajqcue) may be an object similar to SN 2007bi experiencing considerable host galaxy reddening, or an intrinsically long-lived, luminous, and red SN Ic. We analyze the light curves, spectra, and host galaxy properties of these four objects and put them in context of the population of SLSN-I. SN 2018bgv stands out as the fastest-rising SLSN-I observed to date, with a rest-frame g-band rise time of just 10 days from explosion to peak—if it is powered by magnetar spin-down, the implied ejecta mass is only ≃1 M⊙. SN 2018don also displays unusual properties—in addition to its red colors and comparatively massive host galaxy, the light curve undergoes some of the strongest light-curve undulations postpeak seen in an SLSN-I, which we speculate may be due to interaction with circumstellar material. We discuss the promises and challenges of finding SLSNe in large-scale surveys like ZTF given the observed diversity in the population.

Status
Published
Date
November 21st, 2019
Article doi
https://doi.org/10.3847/1538-4357/abaeec
Discussion Paper arxiv
https://arxiv.org/abs/1910.02968
48. The Zwicky Transient Facility Bright Transient Survey I: Spectroscopic Classification and the Redshift Completeness of Local Galaxy Catalogs
Authors
Fremling, C., Miller, A. A., Sharma, Y., Dugas, A., Perley, D. A., Taggart, K., Sollerman, J., Goobar, A., Graham, M. L., Neill, J. D., Nordin, J., Rigault, M., Walters, R., Andreoni, I., Bagdasaryan, A., Belicki, J., Cannella, C., Bellm, Eric C., Cenko, S. B., De, K., Dekany, R., Frederick, S., Golkhou, V. Z., Graham, M. J., Helou, G., Ho, A. Y. Q., Kasliwal, M. M., Kupfer, T., Laher, R. R., Mahabal, A., Masci, F. J., Riddle, R., Rusholme, B., Schulze, S., Shupe, D. L., Smith, R. M., van Velzen, S., Yan, Lin, Yao, Y., Zhuang, Z., Kulkarni, S. R.

The Zwicky Transient Facility (ZTF) is performing a three-day cadence survey of the visible northern sky (~3π) with newly found transient candidates announced via public alerts. The ZTF Bright Transient Survey (BTS) is a large spectroscopic campaign to complement the photometric survey. BTS endeavors to spectroscopically classify all extragalactic transients with m_(peak) ≤ 18.5 mag in either the g_(ZTF) or r_(ZTF) filters, and publicly announce said classifications. BTS discoveries are predominantly supernovae (SNe), making this the largest flux-limited SN survey to date. Here we present a catalog of 761 SNe, classified during the first nine months of ZTF (2018 April 1–2018 December 31). We report BTS SN redshifts from SN template matching and spectroscopic host-galaxy redshifts when available. We analyze the redshift completeness of local galaxy catalogs, the redshift completeness fraction (RCF; the ratio of SN host galaxies with known spectroscopic redshift prior to SN discovery to the total number of SN hosts). Of the 512 host galaxies with SNe Ia, 227 had previously known spectroscopic redshifts, yielding an RCF estimate of 44% ± 4%. The RCF decreases with increasing distance and decreasing galaxy luminosity (for z < 0.05, or ~200 Mpc, RCF ≈ 0.6). Prospects for dramatically increasing the RCF are limited to new multifiber spectroscopic instruments or wide-field narrowband surveys. Existing galaxy redshift catalogs are only ~50% complete at r ≈ 16.9 mag. Pushing this limit several magnitudes deeper will pay huge dividends when searching for electromagnetic counterparts to gravitational wave events or sources of ultra-high-energy cosmic rays or neutrinos.

Status
Published
Date
November 20th, 2019
Article doi
https://doi.org/10.3847/1538-4357/ab8943
Discussion Paper arxiv
https://arxiv.org/abs/1910.12973
47. Characterization of the Nucleus, Morphology and Activity of Interstellar Comet 2I/Borisov by Optical and Near-Infrared GROWTH, Apache Point, IRTF, ZTF and Keck Observations
Authors
Bolin, Bryce T., Lisse, Carey M., Kasliwal, Mansi M., Quimby, Robert M., Tan, Hanjie, Copperwheat, Chris M., Lin, Zhong-Yi, Morbidelli, Alessandro, Abe, Lyu, Bendjoya, Philippe, Burdge, Kevin B., Coughlin, Michael, Fremling, Christoffer, Itoh, Ryosuke, Koss, Michael, Masci, Frank J., Maeno, Syota, Mamajek, Eric E., Marocco, Federico, Murata, Katsuhiro, Rivet, Jean-Pierre, Sitko, Michael L., Stern, Daniel, Vernet, David, Walters, Richard, Yan, Lin, Andreoni, Igor, Bhalerao, Varun, Bodewits, Dennis, De, Kishalay, Deshmukh, Kunal P., Bellm, Eric C., Blagorodnova, Nadejda, Buzasi, Derek, Cenko, S. Bradley, Chang, Chan-Kao, Chojnowski, Drew, Dekany, Richard, Duev, Dmitry A., Graham, Matthew, Jurić, Mario, Kulkarni, Shrinivas R., Kupfer, Thomas, Mahabal, Ashish, Neill, James D., Ngeow, Chow-Choong, Penprase, Bryan, Riddle, Reed, Rodriguez, Hector, Smith, Roger M., Rosnet, Philippe, Sollerman, Jesper, Soumagnac, Maayane T.

We present visible and near-infrared (NIR) photometric and spectroscopic observations of interstellar object (ISO) 2I/Borisov taken from 2019 September 10 to 2019 December 20 using the GROWTH, the Apache Point Observatory Astrophysical Research Consortium 3.5 m, and the NASA Infrared Telescope Facility 3.0 m combined with pre- and postdiscovery observations of 2I obtained by the Zwicky Transient Facility from 2019 March 17 to 2019 May 5. Comparison with imaging of distant solar system comets shows an object very similar to mildly active solar system comets with an outgassing rate of ∼10²⁷ mol s⁻¹. The photometry, taken in filters spanning the visible and NIR range, shows a gradual brightening trend of ~0.03 mag day⁻¹ since 2019 September 10 UTC for a reddish object becoming neutral in the NIR. The light curve from recent and prediscovery data reveals a brightness trend suggesting the recent onset of significant H₂O sublimation with the comet being active with super volatiles such as CO at heliocentric distances >6 au consistent with its extended morphology. Using the advanced capability to significantly reduce the scattered light from the coma enabled by high-resolution NIR images from Keck adaptive optics taken on 2019 October 4, we estimate a diameter for 2I's nucleus of ≾ 1.4 km. We use the size estimates of 1I/'Oumuamua and 2I/Borisov to roughly estimate the slope of the ISO size distribution, resulting in a slope of ~3.4 ± 1.2, similar to solar system comets and bodies produced from collisional equilibrium.

Status
Published
Date
November 20th, 2019
Discussion Paper arxiv
http://arxiv.org/abs/1910.14004
46. Comet 240P/NEAT Is Stirring
Authors
Kelley, Michael S. P., Bodewits, Dennis, Ye, Quanzhi, Farnham, Tony L., Bellm, Eric C., Dekany, Richard, Duev, Dmitry A., Helou, George, Kupfer, Thomas, Laher, Russ R., Masci, Frank J., Prince, Thomas A., Rusholme, Ben, Shupe, David L., Soumagnac, Maayane T., Zolkower, Jeffry

Comets are primitive objects that formed in the protoplanetary disk, and have been largely preserved over the history of the solar system. However, they are not pristine, and surfaces of cometary nuclei do evolve. In order to understand the extent of their primitive nature, we must define the mechanisms that affect their surfaces and comae. We examine the lightcurve of comet 240P/NEAT over three consecutive orbits, and investigate three events of significant brightening (Δm ~ −2 mag). Unlike typical cometary outbursts, each of the three events are long-lived, with enhanced activity for at least 3–6 months. The third event, observed by the Zwicky Transient Facility, occurred in at least two stages. The anomalous behavior appears to have started after the comet was perturbed by Jupiter in 2007, reducing its perihelion distance from 2.53 to 2.12 au. We suggest that the brightening events are temporary transitions to a higher baseline activity level, brought on by the increased insolation, which has warmed previously insulated sub-surface layers. The new activity is isolated to one or two locations on the nucleus, indicating that the surface or immediate sub-surface is heterogeneous. Further study of this phenomenon may provide insight into cometary outbursts, the structure of the near-surface nucleus, and cometary nucleus mantling.

Status
Published
Date
November 18th, 2019
Article doi
https://doi.org/10.3847/2041-8213/ab53e0
Discussion Paper arxiv
https://arxiv.org/abs/1911.02383
45. An Unusual Mid-infrared Flare in a Type 2 AGN: An Obscured Turning-on AGN or Tidal Disruption Event?
Authors
Yang, Qian, Shen, Yue, Liu, Xin, Wu, Xue-Bing, Jiang, Linhua, Shangguan, Jinyi, Graham, Matthew J., Yao, Su

We report the discovery of an exceptional MIR flare in a Type 2 AGN, SDSS J165726.81+234528.1, at z = 0.059. This object brightened by 3 mag in the Wide-field Infrared Survey Explorer (WISE) W1 and W2 bands between 2015 and 2017 (and has been fading since 2018), without significant changes (≾0.2 mag) in the optical over the same period of time. Based on the WISE light curves and near-IR imaging, the flare is more significant at longer wavelengths, suggesting an origin of hot dust emission. The estimated black hole mass (~10^(6.5) M⊙) from different methods places its peak bolometric luminosity around the Eddington limit. The high luminosity of the MIR flare and its multiyear timescale suggest that it most likely originated from reprocessed dust radiation in an extended torus surrounding the AGN, instead of from stellar explosions. The MIR color variability is consistent with known changing-look AGN and tidal disruption events (TDEs), but inconsistent with normal supernovae. We suggest that it is a turning-on Type 2 AGN or TDE, where the optical variability is obscured by the dust torus during the transition. This MIR flare event reveals a population of dramatic nuclear transients that are missed in the optical.

Status
Published
Date
November 6th, 2019
Article doi
https://doi.org/10.3847/1538-4357/ab481a
Discussion Paper arxiv
https://arxiv.org/abs/1907.12721
44. ZChecker: Finding Cometary Outbursts with the Zwicky Transient Facility
Authors
Kelley, Michael S. P., Bodewits, Dennis, Ye, Quanzhi, Laher, Russ R., Masci, Frank J., Monkewitz, Serge, Riddle, Reed, Rusholme, Ben, Shupe, David L., Soumagnac, Maayane T.

ZChecker is new, automated software for finding, measuring, and visualizing known comets in the Zwicky Transient Facility time-domain survey. ZChecker uses on-line ephemeris generation and survey metadata to identify images of targets of interest in the archive. Photometry of each target is measured, and the images processed with temporal filtering to highlight morphological variations in time. Example outputs show outbursts of comets 29P/Schwassmann-Wachmann 1 and 64P/Swift-Gehrels, and an asymmetric coma at C/2017 M4 (ATLAS).

Status
Published
Date
October 17th, 2019
Article pub
http://www.aspbooks.org/a/volumes/article_details/?paper_id=39288
43. SN2018kzr: a rapidly declining transient from the destruction of a white dwarf
Authors
McBrien, Owen, Smart, Stephen J., Chen, Ting-Wan, Inserra, Cosimo, Gillanders, James H., Sim, Stuart A., Jerkstrand, A., Rest, Armin, Valenti, Stefano, Roy, Rupak, Gromadzki, Mariusz, Taubenberger, Stefan, Flors, Andreas, Huber, Mark E., Chambers, Kenneth C., Gal-Yam, A., Young, David R., Nicholl, Matt, Kankare, E., Smith, Ken W., Maguire, K., Mandel, Ilya, Prentice, Simon, Rodriguez, Osmar, Pineda Garcia, Jonathan, Gutierrez, Claudia P., Galbany, Lluis, Barbarino, Cristina, Clark, Peter S. J., Sollerman, Jesper, Kulkarni, S. R., De, Kishalay, Buckley, David, Rau, Arne

We present SN2018kzr, the fastest declining supernova-like transient, second only to the kilonova, AT2017gfo. SN2018kzr is characterized by a peak magnitude of M_r = −17.98, a peak bolometric luminosity of ~1.4 × 10⁴³ erg s⁻¹, and a rapid decline rate of 0.48 ± 0.03 mag day⁻¹ in the r band. The bolometric luminosity evolves too quickly to be explained by pure ⁵⁶Ni heating, necessitating the inclusion of an alternative powering source. Incorporating the spin-down of a magnetized neutron star adequately describes the lightcurve and we estimate a small ejecta mass of M_(ej) = 0.10 ± 0.05 M⊙. Our spectral modeling suggests the ejecta is composed of intermediate mass elements including O, Si, and Mg and trace amounts of Fe-peak elements, which disfavors a binary neutron star merger. We discuss three explosion scenarios for SN2018kzr, given the low ejecta mass, intermediate mass element composition, and high likelihood of additional powering—the core collapse of an ultra-stripped progenitor, the accretion induced collapse (AIC) of a white dwarf, and the merger of a white dwarf and neutron star. The requirement for an alternative input energy source favors either the AIC with magnetar powering or a white dwarf–neutron star merger with energy from disk wind shocks.

Status
Published
Date
October 7th, 2019
Article doi
https://doi.org/10.3847/2041-8213/ab4dae
Discussion Paper arxiv
https://arxiv.org/abs/1909.04545
42. GROWTH on S190425z: Searching Thousands of Square Degrees to Identify an Optical or Infrared Counterpart to a Binary Neutron Star Merger with the Zwicky Transient Facility and Palomar Gattini-IR
Authors
Coughlin, M. W., Ahumada, Tomás, Anand, Shreya, De, Kishalay, Hankins, Matthew J., Kasliwal, Mansi M., Singer, Leo P., Bellm, Eric C., Andreoni, Igor, Cenko, S. B., Cooke, Jeff, Copperwheat, C. M., Dugas, Alison M., Jencson, Jacob E., Perley, D. A., Yu, Po-Chieh, Bhalerao, Varun, Kumar, Harsh, Bloom, Joshua S., Anupama, G. C., Ashley, M., Bagdasaryan, Ashot, Biswas, Rahul, Buckley, D. A. H., Burdge, Kevin B., Cook, David O., Cromer, John, Cunningham, Virginia, D'Aì, A., Dekany, Richard, Delacroix, Alexandre, Dichiara, Simone, Duev, Dmitry A., Dutta, Anirban, Feeney, M., Frederick, Sara, Gatkine, Pradip, Ghosh, Shaon, Goldstein, Daniel A., Golkhou, V. Zach, Goobar, Ariel, Graham, Matthew J., Hanayama, Hidekazu, Horiuchi, Takashi, Hung, Tiara, Jha, Saurabh, Kong, Albert K. H., Giomi, Matteo, Kaplan, David L., Karambelkar, V. R., Kowalski, Marek, Kulkarni, S. R., Kupfer, Thomas, La Parola, V., Masci, Frank J., Mazzali, Paolo A., Moore, Anna M., Mogotsi, Moses, Neill, James D., Ngeow, Chow-Choong, Martínez-Palomera, Jorge, Pavana, M., Ofek, Eran O., Patil, Atharva Sunil, Riddle, Reed, Rigault, Mickael, Rusholme, B., Serabyn, Eugene, Shupe, D. L., Sharma, Yashvi, Sollerman, Jesper, Soon, Jamie, Staats, Kai, Taggart, Kirsty, Tan, Hanjie, Travouillon, T., Troja, Eleonora, Waratkar, Gaurav, Yatsu, Yoichi

The third observing run by LVC has brought the discovery of many compact binary coalescences. Following the detection of the first binary neutron star merger in this run (LIGO/Virgo S190425z), we performed a dedicated follow-up campaign with the Zwicky Transient Facility (ZTF) and Palomar Gattini-IR telescopes. The initial skymap of this single-detector gravitational wave (GW) trigger spanned most of the sky observable from Palomar Observatory. Covering 8000 deg² of the initial skymap over the next two nights, corresponding to 46% integrated probability, ZTF system achieved a depth of ≈21 m_(AB) in g- and r-bands. Palomar Gattini-IR covered 2200 square degrees in J-band to a depth of 15.5 mag, including 32% integrated probability based on the initial skymap. The revised skymap issued the following day reduced these numbers to 21% for the ZTF and 19% for Palomar Gattini-IR. We narrowed 338,646 ZTF transient "alerts" over the first two nights of observations to 15 candidate counterparts. Two candidates, ZTF19aarykkb and ZTF19aarzaod, were particularly compelling given that their location, distance, and age were consistent with the GW event, and their early optical light curves were photometrically consistent with that of kilonovae. These two candidates were spectroscopically classified as young core-collapse supernovae. The remaining candidates were ruled out as supernovae. Palomar Gattini-IR did not identify any viable candidates with multiple detections only after merger time. We demonstrate that even with single-detector GW events localized to thousands of square degrees, systematic kilonova discovery is feasible.

Status
Published
Date
October 7th, 2019
Article doi
https://doi.org/10.3847/2041-8213/ab4ad8
Discussion Paper arxiv
https://arxiv.org/abs/1907.12645
41. Evidence for Late-stage Eruptive Mass Loss in the Progenitor to SN2018gep, a Broad-lined Ic Supernova: Pre-explosion Emission and a Rapidly Rising Luminous Transient
Authors
Ho, Anna Y. Q., Goldstein, Daniel A., Schulze, Steve, Khatami, David K., Perley, Daniel A., Ergon, Mattias, Gal-Yam, Avishay, Corsi, Alessandra, Andreoni, Igor, Barbarino, Cristina, Bellm, Eric C., Blagorodnova, Nadia, Bright, Joe S., Burns, E., Cenko, S. Bradley, Cunningham, Virginia, De, Kishalay, Dekany, Richard, Dugas, Alison, Fender, Rob P., Fransson, Claes, Fremling, Christoffer, Goldstein, Adam, Graham, Matthew J., Hale, David, Horesh, Assaf, Hung, Tiara, Kasliwal, Mansi M., Kuin, N. Paul M., Kulkarni, S. R., Kupfer, Thomas, Lunnan, Ragnhild, Masci, Frank J., Ngeow, Chow-Choong, Nugent, Peter E., Ofek, Eran O., Patterson, Maria T., Petitpas, Glen, Rusholme, Ben, Sai, Hanna, Sfaradi, Itai, Shupe, David L., Sollerman, Jesper, Soumagnac, Maayane T., Tachibana, Yutaro, Taddia, Francesco, Walters, Richard, Wang, Xiaofeng, Yao, Yuhan, Zhang, Xinhan

We present detailed observations of ZTF18abukavn (SN2018gep), discovered in high-cadence data from the Zwicky Transient Facility as a rapidly rising (1.4 ± 0.1 mag/hr) and luminous (M_(g,peak) = −20 mag) transient. It is spectroscopically classified as a broad-lined stripped-envelope supernova (Ic-BL SN). The high peak luminosity (L_(bol) ≳ 3×10^(44) erg sec^(−1)), the short rise time (t_(rise) = 3 days in g-band), and the blue colors at peak (g−r ∼ −0.4) all resemble the high-redshift Ic-BL iPTF16asu, as well as several other unclassified fast transients. The early discovery of SN2018gep (within an hour of shock breakout) enabled an intensive spectroscopic campaign, including the highest-temperature (T_(eff) ≳ 40,000 K) spectra of a stripped-envelope SN. A retrospective search revealed luminous (M_g ∼ M_r ≈ −14mag) emission in the days to weeks before explosion, the first definitive detection of precursor emission for a Ic-BL. We find a limit on the isotropic gamma-ray energy release E_(γ,iso) < 4.9×10^(48) erg, a limit on X-ray emission L_X < 10^(40) erg sec^(−1), and a limit on radio emission νL_ν ≲ 10^(37) erg sec^(−1). Taken together, we find that the early (< 10 days) data are best explained by shock breakout in a massive shell of dense circumstellar material (0.02 M_⊙) at large radii (3×10^(14)cm) that was ejected in eruptive pre-explosion mass-loss episodes. The late-time (>10 days) light curve requires an additional energy source, which could be the radioactive decay of Ni-56.

Status
Published
Date
October 2nd, 2019
Article doi
https://doi.org/10.3847/1538-4357/ab55ec
Discussion Paper arxiv
https://arxiv.org/abs/1904.11009
40. Simultaneous Observations of the Northern TESS Sectors by the Zwicky Transient Facility
Authors
van Roestel, Jan, Bellm, Eric C., Duev, Dmitry A., Fremling, Christoffer, Graham, Matthew J., Masci, Frank, Yan, Lin, Goldstein, Daniel A., Medford, Michael, Ward, Charlotte A., Kulkarni, S. R., Prince, Thomas A.

The Transiting Exoplanet Survey Satellite (TESS) (Schliegel 2017) is a powerful facility for studying a broad range of astrophysical objects. The Zwicky Transient Facility (ZTF) (Bellm et al. 2019; Graham et al. 2019; Masci et al. 2019) is conducting a nightly public survey of all 13 TESS northern sectors in 2019–2020. ZTF will observe the portions of the current TESS sector visible from Palomar Observatory each night. Each ZTF pointing will have one exposure each with g and r filters, totaling two images per night. The first northern sector, Sector 14, was observed from 2019 July 18 to August 15. The observations of the second northern sector, Sector 15, began on 2019 August 15. The majority of Sectors 14 and 15 have been covered by ZTF, except for a portion of TESS Camera 4, due to the visibility limits. ZTF is also making additional nightly g- and r-band observations of denser stellar regions (e.g., near the Galactic Plane) to better facilitate variability studies of Galactic objects.

Status
Published
Date
September 30th, 2019
Article doi
https://doi.org/10.3847/2515-5172/ab459c
Discussion Paper arxiv
https://arxiv.org/abs/1910.04548
39. Introducing the Zwicky Transient Facility and the Be star variability program: a progress report at the National Central University
Authors
Ngeow, C. C., Lee, C. D., Yu, P. C., Masci, F., Laher, R., Kupfer, T., Golkhou, V. Z.

The Zwicky Transient Facility (ZTF) is a modern-day wide-field optical survey to systematically explore the transient and variable sky. The ZTF utilizes the 48-inch Samuel Oschin Schmidt Telescope located at the Palomar Observatory. This telescope is equipped with a mosaic CCD camera that provides a field of view of 47 squared degrees. The allocated observing time of ZTF can be divided into partnership time (40%), public time (40%) and Caltech time (20%). The public time contains two surveys: a 3-day cadence for the Northern Sky Survey and a 1-day cadence for the Galactic Plane Survey. Astronomical communities in South East Asian countries are encouraged to explore the public ZTF data once it is released in March 2019. Taiwan's National Central University (NCU) is one of the partnered institutions, and a major ZTF-related project carried out at NCU is the ZTF Be stars variability (ZTF-BeV) program. The main goal of our program is to study the variability of Be stars in the range of ~13.5 to ~20.5 magnitudes.

Status
Published
Date
September 11th, 2019
Article doi
https://doi.org/10.1088/1742-6596/1231/1/012010
38. Outbursts at Comets 46P/Wirtanen, 64P/Swift-Gehrels, and 78P/Gehrels 2 in 2018
Authors
Kelley, Michael S. P., Bodewits, Dennis, Ye, Quanzhi, Helou, G., Ahumada, Tomás, Laher, Russ R., Masci, Frank J., Ngeow, Chow-Choong, Rusholme, B., Shupe, D. L., Cromer, John, Dekany, Richard

Cometary outbursts are brief increases in the mass-loss rates of comets (Hughes 1990). Most observed outbursts have Δm ≳ 1 mag, but smaller outbursts do occur, such as those observed by the Deep Impact and Rosetta spacecraft at comets 9P/Tempel 1 and 67P/Churyumov-Gerasimenko (A'Hearn et al. 2005; Vincent et al. 2016). The outbursts are smaller than the Deep Impact excavation, suggesting strengths near −0.2 to −1 mag (A'Hearn et al. 2005; Meech et al. 2005; Feldman et al. 2007). Wide-field, time-domain surveys are well-suited to the discovery of cometary outbursts (Graham et al. 2019). We describe initial results from our outburst search using the Zwicky Transient Facility archive (ZTF; Bellm et al. 2019), based on observations of three comets: 46P/Wirtanen, 64P/Swift-Gehrels, and 78P/Gehrels 2. The data are calibrated to the Pan-STARRS 1 photometric system (Masci et al. 2019), and color corrected assuming a reddened solar spectrum (g−r = 0.52 mag, r−i = 0.15 mag). We measured photometry in 7'' radius apertures, then fit and removed a linear trend from each lightcurve: 46P, −0.053 mag day^(−1) (rms 0.02 mag); 64P, −0.053 mag day^(−1) (rms 0.04 mag); 78P, −0.039 mag day^(−1) (rms 0.04 mag).

Status
Published
Date
September 6th, 2019
Publisher doi
https://doi.org/10.3847/2515-5172/ab3fb4
37. GROWTH on GW190425: Searching thousands of square degrees to identify an optical or infrared counterpart to a binary neutron star merger with the Zwicky Transient Facility and Palomar Gattini IR
Authors
Coughlin, Michael W., Ahumada, Tomás, Anand, Shreya, De, Kishalay, Hankins, Matthew J., Kasliwal, Mansi M., Singer, Leo P., Bellm, Eric C., Andreoni, Igor, Cenko, S. Bradley, Cooke, Jeff, Copperwheat, Christopher M., Dugas, Alison M., Jencson, Jacob E., Perley, Daniel A., Yu, Po-Chieh, Bhalerao, Varun B., Kumar, Harsh, Bloom, Joshua S., Anupama, G. C., Ashley, Michael C. B., Bagdasaryan, Ashot, Biswas, Rahul, Buckley, David A. H., Burdge, Kevin B., Cook, David O., Cromer, John, Cunningham, Virginia, D'Aì, Antonino, Dekany, Richard G., Delacroix, Alexandre, Dichiara, Simone, Duev, Dmitry A., Dutta, Anirban, Feeney, Michael, Frederick, Sara, Gatkine, Pradip, Ghosh, Shaon, Goldstein, Daniel A., Golkhou, V. Zach, Goobar, Ariel, Graham, Matthew J., Hanayama, Hidekazu, Horiuchi, Takashi, Hung, Tiara, Jha, Saurabh W., Kong, Albert K. H., Giomi, Matteo, Kaplan, David L., Karambelkar, V. R., Kowalski, Marek, Kulkarni, Shrinivas R., Kupfer, Thomas, La Parola, Valentina, Masci, Frank J., Mazzali, Paolo A., Moore, Anna M., Mogotsi, Moses, Neill, James D., Ngeow, Chow-Choong, Martínez-Palomera, Jorge, Pavana, M., Ofek, Eran O., Patil, Atharva Sunil, Riddle, Reed, Rigault, Mickael, Rusholme, Ben, Serabyn, Eugene, Shupe, David L., Sharma, Yashvi, Sollerman, Jesper, Soon, Jamie, Staats, Kai, Taggart, Kirsty, Tan, Hanjie, Travouillon, Tony, Troja, Eleonora, Waratkar, Gaurav, Yatsu, Yoichi

The beginning of the third observing run by the network of gravitational-wave detectors has brought the discovery of many compact binary coalescences. Prompted by the detection of the first binary neutron star merger in this run (GW190425 / LIGO/Virgo S190425z), we performed a dedicated follow-up campaign with the Zwicky Transient Facility (ZTF) and Palomar Gattini-IR telescopes. As it was a single gravitational-wave detector discovery, the initial skymap spanned most of the sky observable from Palomar Observatory, the site of both instruments. Covering 8000 deg$^2$ of the inner 99\% of the initial skymap over the next two nights, corresponding to an integrated probability of 46\%, the ZTF system achieved a depth of $\approx$\,21 $m_\textrm{AB}$ in $g$- and $r$-bands. Palomar Gattini-IR covered a total of 2200 square degrees in $J$-band to a depth of 15.5\,mag, including 32\% of the integrated probability based on the initial sky map. However, the revised skymap issued the following day reduced these numbers to 21\% for the Zwicky Transient Facility and 19\% for Palomar Gattini-IR. Out of the 338,646 ZTF transient "alerts" over the first two nights of observations, we narrowed this list to 15 candidate counterparts. Two candidates, ZTF19aarykkb and ZTF19aarzaod were particularly compelling given that their location, distance, and age were consistent with the gravitational-wave event, and their early optical lightcurves were photometrically consistent with that of kilonovae. These two candidates were spectroscopically classified as young core-collapse supernovae. The remaining candidates were photometrically or spectroscopically ruled-out as supernovae. Palomar Gattini-IR identified one fast evolving infrared transient after the merger, PGIR19bn, which was later spectroscopically classified as an M-dwarf flare. [abridged]

Status
Submitted
Date
August 2nd, 2019
Discussion Paper arxiv
http://arxiv.org/abs/1907.12645
36. Transient processing and analysis using AMPEL: alert management, photometry, and evaluation of light curves
Authors
Nordin, J., Brinnel, V., van Santen, J., Bulla, M., Feindt, U., Franckowiak, A., Fremling, C., Gal-Yam, A., Giomi, M., Kowalski, M., Mahabal, A., Miranda, N., Rauch, L., Reusch, S., Rigault, M., Schulze, S., Sollerman, J., Stein, R., Yaron, O., van Velzen, S., Ward, C.

Both multi-messenger astronomy and new high-throughput wide-field surveys require flexible tools for the selection and analysis of astrophysical transients. We here introduce the Alert Management, Photometry and Evaluation of Lightcurves (AMPEL) system, an analysis framework designed for high-throughput surveys and suited for streamed data. AMPEL combines the functionality of an alert broker with a generic framework capable of hosting user-contributed code, that encourages provenance and keeps track of the varying information states that a transient displays. The latter concept includes information gathered over time and data policies such as access or calibration levels. We describe a novel ongoing real-time multi-messenger analysis using AMPEL to combine IceCube neutrino data with the alert streams of the Zwicky Transient Facility (ZTF). We also reprocess the first four months of ZTF public alerts, and compare the yields of more than 200 different transient selection functions to quantify efficiencies for selecting Type Ia supernovae that were reported to the Transient Name Server (TNS). We highlight three channels suitable for (1) the collection of a complete sample of extragalactic transients, (2) immediate follow-up of nearby transients and (3) follow-up campaigns targeting young, extragalactic transients. We confirm ZTF completeness in that all TNS supernovae positioned on active CCD regions were detected. AMPEL can assist in filtering transients in real time, running alert reaction simulations, the reprocessing of full datasets as well as in the final scientific analysis of transient data. This text introduces how users can design their own channels for inclusion in the AMPEL live instance that parses the ZTF stream and the real-time submission of high quality extragalactic supernova candidates to the TNS.

Status
Published
Date
August 2nd, 2019
Article doi
https://doi.org/10.1051/0004-6361/201935634
Discussion Paper arxiv
https://arxiv.org/abs/1904.05922
35. Real-bogus classification for the Zwicky Transient Facility using deep learning
Authors
Duev, Dmitry A., Mahabal, Ashish, Masci, Frank J., Graham, Matthew J., Rusholme, Ben, Walters, Richard, Karmarkar, Ishani, Frederick, Sara, Kasliwal, Mansi M., Rebbapragada, Umaa, Ward, Charlotte

Efficient automated detection of flux-transient, re-occurring flux-variable, and moving objects is increasingly important for large-scale astronomical surveys. We present BRAAI, a convolutional-neural-network, deep-learning real/bogus classifier designed to separate genuine astrophysical events and objects from false positive, or bogus, detections in the data of the Zwicky Transient Facility (ZTF), a new robotic time-domain survey currently in operation at the Palomar Observatory in California, USA. BRAAI demonstrates a state-of-the-art performance as quantified by its low false negative and false positive rates. We describe the open-source software tools used internally at Caltech to archive and access ZTF’s alerts and light curves (KOWALSKI ), and to label the data (ZWICKYVERSE). We also report the initial results of the classifier deployment on the Edge Tensor Processing Units that show comparable performance in terms of accuracy, but in a much more (cost-) efficient manner, which has significant implications for current and future surveys.

Status
Published
Date
August 2nd, 2019
Article doi
https://doi.org/10.1093/mnras/stz2357
Discussion Paper arxiv
https://arxiv.org/abs/1907.11259
34. Synthetic Tracking Using ZTF Deep Drilling Data Sets
Authors
Zhai, Chengxing, Ye, Quanzhi, Shao, Michael, Trahan, Russell, Saini, Navtej S., Shen, Janice, Prince, Thomas A., Bellm, Eric C., Graham, Matthew J., Helou, George, Kulkarni, Shrinivas R., Kupfer, Thomas, Laher, Russ R., Mahabal, Ashish, Masci, Frank J., Rusholme, Ben, Rosnet, Philippe, Shupe, David L.

The Zwicky Transit Facility (ZTF) is a powerful time domain survey telescope with a large field of view of 47 deg². We apply the synthetic tracking technique to integrate a ZTF's deep drilling data set, which consists of 133 nominal 30 s exposure frames spanning about 1.5 hr, to search for slowly moving asteroids down to approximately 23rd magnitude. We found 1168 objects from searching 40 of the 64 CCD-quadrant subfields, each of which covers a field size of about 0.73 deg². While most of the objects are in the core region of the asteroid belt, there are asteroids belonging to families of Trojan, Hilda, Hungaria, Phocaea, and near-Earth-asteroids. Such an approach is effective and productive for discovering new asteroids. Here we report the data processing and results as well as discuss a potential deep drilling operation mode using this approach for survey facilities.

Status
Published
Date
August 2nd, 2019
Article doi
https://doi.org/10.1088/1538-3873/ab828b
Discussion Paper arxiv
https://arxiv.org/abs/1907.11299
33. SN 2018fif: The Explosion of a Large Red Supergiant Discovered in Its Infancy by the Zwicky Transient Facility
Authors
Soumagnac, Maayane T., Ganot, Noam, Irani, Ido, Gal-Yam, Avishay, Ofek, Eran O., Waxman, Eli, Morag, Jonathan, Yaron, Ofer, Schulze, Steve, Yang, Yi, Rubin, Adam, Cenko, S. Bradley, Sollerman, Jesper, Perley, Daniel A., Fremling, Christoffer, Nugent, Peter E., Neill, James D., Karamehmetoglu, Emir, Bellm, Eric C., Bruch, Rachel J., Burruss, Rick, Cunningham, Virginia, Dekany, Richard, Golkhou, V. Zach, Graham, Matthew J., Kasliwal, Mansi M., Konidaris, Nicholas P., Kulkarni, Shrinivas R., Kupfer, Thomas, Laher, Russ R., Masci, Frank J., Riddle, Reed, Rigault, Mickael, Rusholme, Ben, van Roestel, Jan, Zackay, Barak

High-cadence transient surveys are able to capture supernovae closer to their first light than ever before. Applying analytical models to such early emission, we can constrain the progenitor stars' properties. In this paper, we present observations of SN 2018fif (ZTF 18abokyfk). The supernova was discovered close to first light and monitored by the Zwicky Transient Facility (ZTF) and the Neil Gehrels Swift Observatory. Early spectroscopic observations suggest that the progenitor of SN 2018fif was surrounded by relatively small amounts of circumstellar material compared to all previous cases. This particularity, coupled with the high-cadence multiple-band coverage, makes it a good candidate to investigate using shock-cooling models. We employ the SOPRANOS code, an implementation of the model by Sapir & Waxman and its extension to early times by Morag et al. Compared with previous implementations, SOPRANOS has the advantage of including a careful account of the limited temporal validity domain of the shock-cooling model as well as allowing usage of the entirety of the early UV data. We find that the progenitor of SN 2018fif was a large red supergiant with a radius of R = 744.0^(+183.0)_(-128.0) R⊙ and an ejected mass of M_(ej) = 9.3^(+0.4)_(-5.8) M⊙. Our model also gives information on the explosion epoch, the progenitor's inner structure, the shock velocity, and the extinction. The distribution of radii is double-peaked, with smaller radii corresponding to lower values of the extinction, earlier recombination times, and a better match to the early UV data. If these correlations persist in future objects, denser spectroscopic monitoring constraining the time of recombination, as well as accurate UV observations (e.g., with ULTRASAT), will help break the extinction/radius degeneracy and independently determine both.

Status
Published
Date
August 2nd, 2019
Article doi
https://doi.org/10.3847/1538-4357/abb247
Discussion Paper arxiv
https://arxiv.org/abs/1907.11252
32. Fully automated integral field spectrograph pipeline for the SEDMachine: pysedm
Authors
Rigault, M., Neill, J. D., Blagorodnova, N., Dugas, A., Feeney, M., Walters, R., Brinnel, V., Copin, Y., Fremling, C., Nordin, J., Sollerman, J.

Current time domain facilities are discovering hundreds of new galactic and extra-galactic transients every week. Classifying the ever-increasing number of transients is challenging, yet crucial to furthering our understanding of their nature, discovering new classes, and ensuring sample purity, for instance, for Supernova Ia cosmology. The Zwicky Transient Facility is one example of such a survey. In addition, it has a dedicated very-low resolution spectrograph, the SEDMachine, operating on the Palomar 60-inch telescope. This spectrograph’s primary aim is object classification. In practice most, if not all, transients of interest brighter than ∼19 mag are typed. This corresponds to approximately 10–15 targets a night. In this paper, we present a fully automated pipeline for the SEDMachine. This pipeline has been designed to be fast, robust, stable and extremely flexible. PYSEDM enables the fully automated spectral extraction of a targeted point source object in less than five minutes after the end of the exposure. The spectral color calibration is accurate at the few percent level. In the 19 weeks since PYSEDM entered production in early August of 2018, we have classified, among other objects, about 400 Type Ia supernovae and 140 Type II supernovae. We conclude that low resolution, fully automated spectrographs such as the “SEDMachine with pysedm” installed on 2-m class telescopes within the southern hemisphere could allow us to automatically and simultaneously type and obtain a redshift for most (if not all) bright transients detected by LSST within z <  0.2, notably potentially all Type Ia Supernovae. In comparison with the current SEDM design, this would require higher spectral resolution (R ≳ 1000) and slightly improved throughput. With this perspective in mind, pysedm is designed to easily be adaptable to any IFU-like spectrograph.

Status
Published
Date
July 25th, 2019
Article doi
https://doi.org/10.1051/0004-6361/201935344
Discussion Paper arxiv
https://arxiv.org/abs/1902.08526
31. General relativistic orbital decay in a seven-minute-orbital-period eclipsing binary system
Authors
Burdge, Kevin B., Coughlin, Michael W., Fuller, Jim, Kupfer, Thomas, Bellm, Eric C., Bildsten, Lars, Graham, Matthew J., Kaplan, David L., van Roestel, Jan, Dekany, Richard G., Duev, Dmitry A., Feeney, Michael, Giomi, Matteo, Helou, George, Kaye, Stephen, Laher, Russ R., Mahabal, Ashish A., Masci, Frank J., Riddle, Reed, Shupe, David L., Soumagnac, Maayane T., Smith, Roger M., Szkody, Paula, Walters, Richard, Kulkarni, Shrinivas R., Prince, Thomas A.

General relativity predicts that short-orbital-period binaries emit considerable amounts of gravitational radiation. The upcoming Laser Interferometer Space Antenna (LISA) is expected to detect tens of thousands of such systems but few have been identified4, of which only one5 is eclipsing—the double-white-dwarf binary SDSS J065133.338+284423.37, which has an orbital period of 12.75 minutes. Here we report the discovery of an eclipsing double-white-dwarf binary system, ZTF J153932.16+502738.8, with an orbital period of 6.91 minutes. This system has an orbit so compact that the entire binary could fit within the diameter of the planet Saturn. The system exhibits a deep eclipse, and a double-lined spectroscopic nature. We see rapid orbital decay, consistent with that expected from general relativity. ZTF J153932.16+502738.8 is a strong source of gravitational radiation close to the peak of LISA’s sensitivity, and we expect it to be detected within the first week of LISA observations, once LISA launches in approximately 2034.

Status
Published
Date
July 24th, 2019
Article doi
https://doi.org/10.1038/s41586-019-1403-0
Free ReadCube access pub
https://rdcu.be/bLzhJ
Discussion Paper arxiv
https://arxiv.org/abs/1907.11291
30. Discovery of an Intermediate-luminosity Red Transient in M51 and Its Likely Dust-obscured, Infrared-variable Progenitor
Authors
Jencson, Jacob E., Adams, Scott M., Bond, Howard E., Van Dyk, Schuyler D., Kasliwal, Mansi M., Bally, John, Blagorodnova, Nadja, De, Kishalay, Fremling, Christoffer, Yao, Yuhan, Fruchter, A., Rubin, David, Barbarino, Cristina, Sollerman, J., Miller, Adam A., Hicks, Erin K. S., Malkan, Matthew, Andreoni, Igor, Bellm, Eric C., Buchheim, Robert, Dekany, Richard, Feeney, Michael, Frederick, Sara, Gal-Yam, A., Gehrz, R. D., Giomi, Matteo, Graham, Matthew J., Green, Wayne, Hale, David, Hankins, Matthew J., Hanson, Mark, Helou, George, Ho, Anna Y. Q., Hung, T., Jurić, Mario, Kendurkar, Malhar R., Kulkarni, S. R., Lau, Ryan M., Masci, Frank J., Neill, James D., Quin, Kevin, Riddle, Reed, Rusholme, B., Sims, Forrest, Smith, Nathan, Smith, Roger M., Soumagnac, Maayane T., Tachibana, Yutaro, Tinyanont, Samaporn, Walters, Richard, Watson, Stanley, Williams, Robert E.

We present the discovery of an optical transient (OT) in Messier 51, designated M51 OT2019-1 (also ZTF 19aadyppr, AT 2019abn, ATLAS19bzl), by the Zwicky Transient Facility (ZTF). The OT rose over 15 days to an observed luminosity of M_r = −13 (νL ν = 9 × 10^6 L_⊙), in the luminosity gap between novae and typical supernovae (SNe). Spectra during the outburst show a red continuum, Balmer emission with a velocity width of ≈400 km s^(−1), Ca II and [Ca II] emission, and absorption features characteristic of an F-type supergiant. The spectra and multiband light curves are similar to the so-called "SN impostors" and intermediate-luminosity red transients (ILRTs). We directly identify the likely progenitor in archival Spitzer Space Telescope imaging with a 4.5 μm luminosity of M_([4.5]) ≈ −12.2 mag and a [3.6]–[4.5] color redder than 0.74 mag, similar to those of the prototype ILRTs SN 2008S and NGC 300 OT2008-1. Intensive monitoring of M51 with Spitzer further reveals evidence for variability of the progenitor candidate at [4.5] in the years before the OT. The progenitor is not detected in pre-outburst Hubble Space Telescope optical and near-IR images. The optical colors during outburst combined with spectroscopic temperature constraints imply a higher reddening of E(B − V) ≈ 0.7 mag and higher intrinsic luminosity of M_r ≈ −14.9 mag (νL_ν = 5.3 × 10^7 L⊙) near peak than seen in previous ILRT candidates. Moreover, the extinction estimate is higher on the rise than on the plateau, suggestive of an extended phase of circumstellar dust destruction. These results, enabled by the early discovery of M51 OT2019-1 and extensive pre-outburst archival coverage, offer new clues about the debated origins of ILRTs and may challenge the hypothesis that they arise from the electron-capture induced collapse of extreme asymptotic giant branch stars.

Status
Published
Date
June 19th, 2019
Article doi
https://doi.org/10.3847/2041-8213/ab2c05
Discussion Paper arxiv
https://arxiv.org/abs/1904.07857
29. A New Class of Changing-Look LINERs
Authors
Frederick, Sara, Gezari, Suvi, Graham, Matthew J., Cenko, S. Bradley, van Velzen, Sjoert, Stern, Daniel, Blagorodnova, Nadejda, Kulkarni, Shrinivas R., Yan, Lin, De, Kishalay, Fremling, U. Christoffer, Hung, Tiara, Kara, Erin, Shupe, David L., Ward, Charlotte, Bellm, Eric C., Dekany, Richard, Duev, Dmitry A., Feindt, Ulrich, Giomi, Matteo, Kupfer, Thomas, Laher, Russ R., Masci, Frank J., Miller, Adam A., Ngeow, Chow-Choong, Patterson, Maria T., Porter, Michael, Rusholme, Ben, Sollerman, Jesper, Walters, Richard, Neill, James D.

We report the discovery of six active galactic nuclei (AGNs) caught "turning on" during the first nine months of the Zwicky Transient Facility (ZTF) survey. The host galaxies were classified as low-ionization nuclear emission-line region galaxies (LINERs) by weak narrow forbidden line emission in their archival SDSS spectra, and detected by ZTF as nuclear transients. In five of the cases, we found via follow-up spectroscopy that they had transformed into broad-line AGNs, reminiscent of the changing-look LINER iPTF16bco. In one case, ZTF18aajupnt/AT2018dyk, follow-up Hubble Space Telescope ultraviolet and ground-based optical spectra revealed the transformation into a narrow-line Seyfert 1 with strong [Fe vii, x, xiv] and He ii λ 4686 coronal lines. Swift monitoring observations of this source reveal bright UV emission that tracks the optical flare, accompanied by a luminous soft X-ray flare that peaks ~60 days later. Spitzer follow-up observations also detect a luminous mid-infrared flare, implying a large covering fraction of dust. Archival light curves of the entire sample from CRTS, ATLAS, and ASAS-SN constrain the onset of the optical nuclear flaring from a prolonged quiescent state. Here we present the systematic selection and follow-up of this new class of changing-look LINERs, compare their properties to previously reported changing-look Seyfert galaxies, and conclude that they are a unique class of transients well-suited to test the uncertain physical processes associated with the LINER accretion state.

Status
Published
Date
June 19th, 2019
Article doi
https://doi.org/10.3847/1538-4357/ab3a38
Discussion Paper arxiv
https://arxiv.org/abs/1904.10973
28. The Death Throes of a Stripped Massive Star: An Eruptive Mass-Loss History Encoded in Pre-Explosion Emission, a Rapidly Rising Luminous Transient, and a Broad-Lined Ic Supernova SN2018gep
Authors
Ho, Anna Y. Q., Goldstein, Daniel A., Schulze, Steve, Khatami, David K., Perley, D. A., Ergon, Mattias, Gal-Yam, Avishay, Corsi, Alessandra, Andreoni, Igor, Barbarino, Cristina, Bellm, Eric C., Blagorodnova, Nadejda, Bright, Joe S., Burns, Eric, Cenko, S. B., Cunningham, Virginia, De, Kishalay, Dekany, Richard, Dugas, Alison M., Fender, Rob P., Fransson, Claes, Fremling, Christoffer, Goldstein, Adam, Graham, Matthew J., Hale, David, Horesh, Assaf, Hung, Tiara, Kasliwal, Mansi M., Kuin, N. Paul M., Kulkarni, S. R., Kupfer, Thomas, Lunnan, Ragnhild, Masci, Frank J., Ngeow, Chow-Choong, Nugent, Peter E., Ofek, Eran O., Patterson, Maria T., Pettipas, Glen, Rusholme, Ben, Sai, Hanna, Sfaradi, Itai, Shupe, D. L., Sollerman, J., Soumagnac, Maayane T., Tachibana, Yutaro, Taddia, Francesco, Walters, Richard, Wang, Xiaofeng, Yao, Yuhan, Zhang, Xinhan

We present detailed observations of ZTF18abukavn (SN2018gep), discovered in high-cadence data from the Zwicky Transient Facility as a rapidly rising (1.3 mag/hr) and luminous (M_(g,peak) = −20 mag) transient. It is spectroscopically classified as a broad-lined stripped-envelope supernova (Ic-BL SN). The rapid rise to peak bolometric luminosity and blue colors at peak (t_(rise)∼0.5-3 days, L_(bol)≳3×10^(44) erg sec^(−1), g−r = −0.3) resemble the high-redshift Ic-BL iPTF16asu, as well as several other unclassified fast transients. The early discovery of SN2018gep (within an hour of shock breakout) enabled an intensive spectroscopic campaign, including the highest-temperature (T_(eff) ≳ 40,000K) spectra of a stripped-envelope SN. A retrospective search revealed luminous (M_g ∼ M_r ≈ −14mag) emission in the days to weeks before explosion, the first definitive detection of precursor emission for a Ic-BL. We find a limit on the isotropic gamma-ray energy release E_(γ,iso) < 4.9×10^(48) erg, a limit on X-ray emission L_X < 10^(40) erg sec^(−1), and a limit on radio emission νL_ν ≲ 10^(37) erg sec^(−1). Taken together, we find that the data are best explained by shock breakout in a massive shell of dense circumstellar material (0.02 M⊙) at large radii (3×10^(14)cm) that was ejected in eruptive pre-explosion mass-loss episodes.

Status
Unpublished
Date
June 18th, 2019
Discussion Paper arxiv
https://arxiv.org/abs/1904.11009
27. Discovery of Highly Blueshifted Broad Balmer and Metastable Helium Absorption Lines in a Tidal Disruption Event
Authors
Hung, T., Cenko, S. B., Roth, Nathaniel, Gezari, S., Veilleux, S., van Velzen, Sjoert, Gaskell, C. Martin, Foley, Ryan J., Blagorodnova, N., Yan, Lin, Graham, M. J., Brown, J. S., Siebert, M. R., Frederick, Sara, Ward, Charlotte, Gatkine, Pradip, Gal-Yam, Avishay, Yang, Yi, Schulze, S., Dimitriadis, G., Kupfer, Thomas, Shupe, David L., Rusholme, Ben, Masci, Frank J., Riddle, Reed, Soumagnac, Maayane T., van Roestel, J., Dekany, Richard

We report the discovery of nonstellar hydrogen Balmer and metastable helium absorption lines accompanying a transient, high-velocity (0.05c) broad absorption line (BAL) system in the optical spectra of the tidal disruption event (TDE) AT2018zr (z = 0.071). In the Hubble Space Telescope UV spectra, absorption of high- and low-ionization lines is also present at this velocity, making AT2018zr resemble a low-ionization BALQSO. We conclude that these transient absorption features are more likely to arise in fast outflows produced by the TDE than absorbed by the unbound debris. In accordance with the outflow picture, we are able to reproduce the flat-topped Hα emission in a spherically expanding medium without invoking the typical prescription of an elliptical disk. We also report the appearance of narrow (~1000 kms^(−1)) N III λ4640, He II λ4686, Hα, and Hβ emission in the late-time optical spectra of AT2018zr, which may be a result of UV continuum hardening at late times, as observed by Swift. Including AT2018zr, we find a high association rate (three out of four) of BALs in the UV spectra of TDEs. This suggests that outflows may be ubiquitous among TDEs and less sensitive to viewing angle effects compared to QSO outflows.

Status
Published
Date
June 18th, 2019
Article doi
https://doi.org/10.3847/1538-4357/ab24de
Discussion Paper arxiv
https://arxiv.org/abs/1903.05637
26. A New Class of Large-amplitude Radial-mode Hot Subdwarf Pulsators
Authors
Kupfer, Thomas, Bauer, Evan B., Burdge, Kevin B., Bellm, Eric C., Bildsten, Lars, Fuller, Jim, Hermes, J. J., Kulkarni, Shrinivas R., Prince, Thomas A., van Roestel, Jan, Dekany, Richard, Duev, Dmitry A., Feeney, Michael, Giomi, Matteo, Graham, Matthew J., Kaye, Stephen, Laher, Russ R., Masci, Frank J., Porter, Michael, Riddle, Reed, Shupe, David L., Smith, Roger M., Soumagnac, Maayane T., Szkody, Paula, Ward, Charlotte

Using high-cadence observations from the Zwicky Transient Facility at low Galactic latitudes, we have discovered a new class of pulsating, hot compact stars. We have found four candidates, exhibiting blue colors (g − r ≤ −0.1 mag), pulsation amplitudes of >5%, and pulsation periods of 200–475 s. Fourier transforms of the light curves show only one dominant frequency. Phase-resolved spectroscopy for three objects reveals significant radial velocity, T_(eff), and log(g) variations over the pulsation cycle, which are consistent with large-amplitude radial oscillations. The mean T_(eff) and log(g) for these stars are consistent with hot subdwarf B (sdB) effective temperatures and surface gravities. We calculate evolutionary tracks using MESA and adiabatic pulsations using GYRE for low-mass, helium-core pre-white dwarfs (pre-WDs) and low-mass helium-burning stars. Comparison of low-order radial oscillation mode periods with the observed pulsation periods show better agreement with the pre-WD models. Therefore, we suggest that these new pulsators and blue large-amplitude pulsators (BLAPs) could be members of the same class of pulsators, composed of young ≈0.25–0.35 M_⊙ helium-core pre-WDs.

Status
Published
Date
June 17th, 2019
Article doi
https://doi.org/10.3847/2041-8213/ab263c
Discussion Paper arxiv
https://arxiv.org/abs/1906.00979
25. ZTF18aalrxas: A Type IIb Supernova from a Very Extended Low-mass Progenitor
Authors
Fremling, C., Ko, H., Dugas, A., Ergon, M., Sollerman, J., Bagdasaryan, A., Barbarino, C., Belicki, Justin, Bellm, Eric C., Blagorodnova, N., De, Kishalay, Dekany, Richard, Frederick, S., Gal-Yam, A., Goldstein, Daniel A., Golkhou, V. Zach, Graham, Matthew J., Kasliwal, Mansi M., Kowalski, M., Kulkarni, S. R., Kupfer, T., Laher, Russ R., Masci, Frank J., Miller, A. A., Neill, J. D., Perley, D. A., Rebbapragada, U. D., Riddle, Reed, Rusholme, B., Schulze, S., Smith, R. M., Tartaglia, L., Yan, Lin, Yao, Y.

We investigate ZTF18aalrxas, a double-peaked Type IIb core-collapse supernova (SN) discovered during science validation of the Zwicky Transient Facility. ZTF18aalrxas was discovered while the optical emission was still rising toward the initial cooling peak (0.7 mag over 2 days). Our observations consist of multi-band (ultraviolet and optical) light curves (LCs), and optical spectra spanning from ≈0.7 to ≈180 days past the explosion. We use a Monte-Carlo based non-local thermodynamic equilibrium model that simultaneously reproduces both the ^(56)Ni-powered bolometric LC and our nebular spectrum. This model is used to constrain the synthesized radioactive nickel mass (0.17 M☉) and the total ejecta mass (1.7 M☉) of the SN. The cooling emission is modeled using semi-analytical extended envelope models to constrain the progenitor radius (790–1050 R ⊙) at the time of explosion. Our nebular spectrum shows signs of interaction with a dense circumstellar medium (CSM), and this spectrum is modeled and analyzed to constrain the amount of ejected oxygen (0.3–0.5 M☉) and the total hydrogen mass (≈0.15 M☉) in the envelope of the progenitor. The oxygen mass of ZTF18aalrxas is consistent with a low (12–13 M☉) zero-age main-sequence mass progenitor. The LCs and spectra of ZTF18aalrxas are not consistent with massive single-star SN Type IIb progenitor models. The presence of an extended hydrogen envelope of low mass, the presence of a dense CSM, the derived ejecta mass, and the late-time oxygen emission can all be explained in a binary model scenario.

Status
Published
Date
June 6th, 2019
Article doi
https://doi.org/10.3847/2041-8213/ab218f
Discussion Paper arxiv
https://arxiv.org/abs/1903.09262
24. Novel Measures for Rare Transients
Authors
Mahabal, Ashish

Data volumes in astronomy have been growing rapidly. Various projects and methodologies are starting to deal with this. As we cross-match and correlate datasets, the number of parameters per object—in other words dimensions we need to deal with— is also growing. This leads to more interesting issues as many values are missing, and many parameters are non-homogeneously redundant. One needs to tease apart clusters in this space which represent different physical properties, and hence phenomena. We describe measures that help to do that for transients from the Catalina Realtime Transient Survey, and project it to near future surveys. The measures are based partly on domain knowledge and are incorporated into statistical and machine learning techniques. We also describe the discriminating role of appropriate followup observations in near-real-time classification of transients. In particular such novel measures will help us find relatively rare transients.

Status
Published
Date
May 3rd, 2019
Article author
http://www.slac.stanford.edu/econf/C131113.1/papers/mahabal.pdf
23. The Zwicky Transient Facility
Authors
Bellm, Eric

The Zwicky Transient Facility (ZTF) is a next-generation optical synoptic survey that builds on the experience and infrastructure of the Palomar Transient Factory (PTF). Using a new 47 deg2 survey camera, ZTF will survey more than an order of magnitude faster than PTF to discover rare transients and variables. I describe the survey and the camera design. Searches for young supernovae, fast transients, counterparts to gravitational-wave detections, and rare variables will benefit from ZTF's high cadence, wide area survey.

Status
Published
Date
May 1st, 2019
Article pub
http://www.slac.stanford.edu/econf/C131113.1/papers/bellm.pdf
Discussion Paper arxiv
http://arxiv.org/abs/1410.8185
22. simsurvey: Estimating Transient Discovery Rates for the Zwicky Transient Facility
Authors
Feindt, Ulrich, Nordin, Jakob, Rigault, Mickael, Brinnel, Valéry, Dhawan, Suhail, Goobar, Ariel, Kowalski, Marek

When planning a survey for astronomical transients, many factors such as cadence, filter choice, sky coverage, and depth of observations need to be balanced in order to optimize the scientific gain of the survey. Here we present a software package called simsurvey for simulating the supernova lightcurves that are expected based on a survey strategy, which can then be used to determine the potential for discoveries of each strategy in question. The code is set up in a modular fashion that allows easy modification of small details of the survey and enables the user to adapt it to any survey design and transient template that they wish to use in planning their survey. As an example of its utility, we use simsurvey to simulate the lightcurve of several types of supernovae that the recently started Zwicky Transient Facility (ZTF) is expected to find and compare the results to the discoveries made during its early operations. We conclude that ZTF will find thousands of bright supernovae per year, of which about 10 could potentially be found with two days of explosion. Over the course of three years the survey will obtain lightcurves of about 1800 type Ia supernovae with z < 0.1 that can be used as distance indicators in cosmology if they are spectroscopically classified using additional telescopes. In a comparison to detections from the ZTF public survey, we found good agreement with the numbers of detections expected from the simulations.

Status
Unpublished
Date
May 1st, 2019
Discussion Paper arxiv
http://arxiv.org/abs/1902.03923
21. The Zwicky Transient Facility: Surveys and Scheduler
Authors
Bellm, Eric C., Kulkarni, Shrinivas R., Barlow, Tom, Feindt, Ulrich, Graham, Matthew J., Goobar, Ariel, Kupfer, Thomas, Ngeow, Chow-Choong, Nugent, Peter, Ofek, Eran, Prince, Thomas A., Riddle, Reed, Walters, Richard, Ye, Quan-Zhi

We present a novel algorithm for scheduling the observations of time-domain imaging surveys. Our integer linear programming approach optimizes an observing plan for an entire night by assigning targets to temporal blocks, enabling strict control of the number of exposures obtained per field and minimizing filter changes. A subsequent optimization step minimizes slew times between each observation. Our optimization metric self-consistently weights contributions from time-varying airmass, seeing, and sky brightness to maximize the transient discovery rate. We describe the implementation of this algorithm on the surveys of the Zwicky Transient Facility and present its on-sky performance.

Status
Published
Date
May 1st, 2019
Article doi
https://doi.org/10.1088/1538-3873/ab0c2a
Discussion Paper arxiv
https://arxiv.org/abs/1905.02209
20. Towards Efficient Detection of Small Near-Earth Asteroids Using the Zwicky Transient Facility (ZTF)
Authors
Ye, Quanzhi, Masci, Frank J., Lin, Hsing Wen, Bolin, Bryce, Chang, Chan-Kao, Duev, Dmitry A., Helou, George, Ip, Wing-Huen, Kaplan, David L., Kramer, Emily, Mahabal, Ashish, Ngeow, Chow-Choong, Nielsen, Avery J., Prince, Thomas A., Tan, Hanjie, Yeh, Ting-Shuo, Bellm, Eric C., Dekany, Richard, Giomi, Matteo, Graham, Matthew J., Kulkarni, Shrinivas R., Kupfer, Thomas, Laher, Russ R., Rusholme, Ben, Shupe, David L., Ward, Charlotte

We describe ZStreak, a semi-real-time pipeline specialized in detecting small, fast-moving, near-Earth asteroids (NEAs), which is currently operating on the data from the newly commissioned Zwicky Transient Facility (ZTF) survey. Based on a prototype originally developed by Waszczak et al. (2017) for the Palomar Transient Factory (PTF), the predecessor of ZTF, ZStreak features an improved machine-learning model that can cope with the 10× data rate increment between PTF and ZTF. Since its first discovery on 2018 February 5 (2018 CL), ZTF/ZStreak has discovered 45 confirmed new NEAs over a total of 232 observable nights until 2018 December 31. Most of the discoveries are small NEAs, with diameters less than ~100 m. By analyzing the discovery circumstances, we find that objects having the first to last detection time interval under 2 hr are at risk of being lost. We will further improve real-time follow-up capabilities, and work on suppressing false positives using deep learning.

Status
Published
Date
May 1st, 2019
Article doi
https://doi.org/10.1088/1538-3873/ab1b18
Discussion Paper arxiv
https://arxiv.org/abs/1904.09645
19. DeepStreaks: identifying fast-moving objects in the Zwicky Transient Facility data with deep learning
Authors
Duev, Dmitry A., Mahabal, Ashish, Ye, Quanzhi, Tirumala, Kushal, Belicki, Justin, Dekany, Richard, Frederick, Sara, Graham, Matthew J., Laher, Russ R., Masci, Frank J., Prince, Thomas A., Riddle, Reed, Rosnet, Philippe, Soumagnac, Maayane T.

We present DeepStreaks, a convolutional-neural-network, deep-learning system designed to efficiently identify streaking fast-moving near-Earth objects that are detected in the data of the Zwicky Transient Facility (ZTF), a wide-field, time-domain survey using a dedicated 47 deg2 camera attached to the Samuel Oschin 48-inch Telescope at the Palomar Observatory in California, United States. The system demonstrates a 96-98% true positive rate, depending on the night, while keeping the false positive rate below 1%. The sensitivity of DeepStreaks is quantified by the performance on the test data sets as well as using known near-Earth objects observed by ZTF. The system is deployed and adapted for usage within the ZTF Solar-System framework and has significantly reduced human involvement in the streak identification process, from several hours to typically under 10 minutes per day.

Status
Published
Date
May 1st, 2019
Article doi
https://doi.org/10.1093/mnras/stz1096
Discussion Paper arxiv
https://arxiv.org/abs/1904.05920
DeepStreaks code and pre-trained models related
https://github.com/dmitryduev/DeepStreaks
18. Processing Images from the Zwicky Transient Facility
Authors
Laher, Russ R., Masci, Frank J., Groom, Steve, Rusholme, Benjamin, Shupe, David L., Jackson, Ed, Surace, Jason, Flynn, Dave, Landry, Walter, Terek, Scott, Helou, George, Beck, Ron, Hasscopians, Eugean, Rebbapragada, Umaa, Bue, Brian, Smith, Roger M., Dekany, Richard G., Miller, Adam A., Cenko, S. B., Bellm, Eric, Patterson, Maria, Kupfer, Thomas, Yan, Lin, Barlow, Tom, Graham, Matthew, Kasliwal, Mansi M., Prince, Thomas A., Kulkarni, Shrinivas R.

The Zwicky Transient Facility is a new robotic-observing program, in which a newly engineered 600-MP digital camera with a pioneeringly large field of view, 47 square degrees, will be installed into the 48-inch Samuel Oschin Telescope at the Palomar Observatory. The camera will generate ~1 petabyte of raw image data over three years of operations. In parallel related work, new hardware and software systems are being developed to process these data in real time and build a long-term archive for the processed products. The first public release of archived products is planned for early 2019, which will include processed images and astronomical-source catalogs of the northern sky in the g and r bands. Source catalogs based on two different methods will be generated for the archive: aperture photometry and point-spread-function fitting.

Status
Published
Date
April 18th, 2019
Article doi
https://doi.org/10.32374/rtsre.2017.031
Discussion Paper arxiv
https://arxiv.org/abs/1708.01584
17. Multiple Outbursts of Asteroid (6478) Gault
Authors
Ye, Quanzhi, Kelley, Michael S. P., Bodewits, Dennis, Bolin, Bryce, Jones, Lynne, Lin, Zhong-Yi, Bellm, Eric C., Dekany, Richard, Duev, Dmitry A., Groom, Steven, Helou, George, Kulkarni, Shrinivas R., Kupfer, Thomas, Masci, Frank J., Prince, Thomas A., Soumagnac, Maayane T.

Main-belt asteroid (6478) Gault unexpectedly sprouted two tails in late 2018 and early 2019, identifying it as a new active asteroid. Here we present observations obtained by the 1.2 m Zwicky Transient Facility survey telescope that provide detailed time-series coverage of the onset and evolution of Gault's activity. Gault exhibited two brightening events, with the first one starting on 2018 October 18 ± 5 days and a second one starting on 2018 December 24 ± 1 days. The amounts of mass released are 2 × 10^7 kg and 1 × 10^6 kg, respectively. Based on photometric measurements, each event persisted for about a month. Gault's color has not changed appreciably over time, with a pre-outburst color of g_(PS1) − r_(PS1) = 0.50 ± 0.04 and g_(PS1) − r_(PS1) = 0.46 ± 0.04 during the two outbursts. Simulations of dust dynamics shows that the ejecta consists of dust grains of up to 10 μm in size that are ejected at low velocities below 1 m s^(-1) regardless of particle sizes. This is consistent with non-sublimation-driven ejection events. The size distribution of the dust exhibits a broken power law, with particles at 10–20 μm following a power law of −2.5 to −3.0, while larger particles follow a steeper slope of −4.0. The derived properties can be explained by either rotational excitation of the nucleus or a merger of a near-contact binary, with the latter scenario to be statistically more likely.

Status
Published
Date
March 29th, 2019
Article doi
https://doi.org/10.3847/2041-8213/ab0f3c
Discussion Paper arxiv
https://arxiv.org/abs/1903.05320
16. ZTF 18aaqeasu (SN2018byg): A Massive Helium-shell Double Detonation on a Sub-Chandrasekhar-mass White Dwarf
Authors
De, Kishalay, Kasliwal, Mansi M., Polin, Abigail, Nugent, P. E., Bildsten, Lars, Adams, Scott M., Bellm, Eric C., Blagorodnova, N., Burdge, Kevin B., Cannella, Christopher, Cenko, S. B., Dekany, Richard, Feeney, Michael, Hale, David, Fremling, U. Christoffer, Graham, Matthew J., Ho, Anna Y. Q., Jencson, Jacob E., Kulkarni, S. R., Laher, Russ R., Masci, Frank J., Miller, Adam A., Patterson, Maria T., Rebbapragada, Umaa, Riddle, Reed L., Shupe, D. L., Smith, Roger M.

The detonation of a helium shell on a white dwarf (WD) has been proposed as a possible explosion triggering mechanism for SNe Ia. Here, we report ZTF 18aaqeasu (SN 2018byg/ATLAS 18pqq), a peculiar Type I supernova, consistent with being a helium-shell double-detonation. With a rise time of ≈18 days from explosion, the transient reached a peak absolute magnitude of M_R ≈ −18.2 mag, exhibiting a light curve akin to sub-luminous SN 1991bg-like SNe Ia, albeit with an unusually steep increase in brightness within a week from explosion. Spectra taken near peak light exhibit prominent Si absorption features together with an unusually red color (g − r ≈ 2 mag) arising from nearly complete line blanketing of flux blueward of 5000 Å. This behavior is unlike any previously observed thermonuclear transient. Nebular phase spectra taken at and after ≈30 days from peak light reveal evidence of a thermonuclear detonation event dominated by Fe-group nucleosynthesis. We show that the peculiar properties of ZTF 18aaqeasu are consistent with the detonation of a massive (≈0.15 M⊙) helium shell on a sub-Chandrasekhar mass (≈0.75 M⊙) WD after including mixing of ≈0.2 M⊙ of material in the outer ejecta. These observations provide evidence of a likely rare class of thermonuclear supernovae arising from detonations of massive helium shells.

Status
Published
Date
March 14th, 2019
Article doi
https://doi.org/10.3847/2041-8213/ab0aec
Discussion Paper arxiv
https://arxiv.org/abs/1901.00874
15. The First Tidal Disruption Flare in ZTF: From Photometric Selection to Multi-wavelength Characterization
Authors
van Velzen, Sjoert, Gezari, Suvi, Cenko, S. B., Kara, E., Miller-Jones, J. C. A., Hung, Tiara, Bright, Joe S., Roth, Nathaniel, Blagorodnova, N., Huppenkothen, Daniela, Yan, Lin, Ofek, Eran O., Sollerman, J., Frederick, Sara, Ward, Charlotte, Graham, Matthew J., Fender, Robert, Kasliwal, Mansi M., Canella, Chris, Stein, Robert, Giomi, Matteo, Brinnel, Valery, van Santen, J., Nordin, Jakob, Bellm, Eric C., Dekany, Richard, Fremling, Christoffer, Golkhou, V. Zach, Kupfer, Thomas, Kulkarni, Shrinivas R., Laher, Russ R., Mahabal, Ashish, Masci, Frank J., Miller, Adam A., Neill, James D., Riddle, Reed, Rigault, Mickael, Rusholme, B., Soumagnac, Maayane T., Tachibana, Yutaro

We present Zwicky Transient Facility (ZTF) observations of the tidal disruption flare AT2018zr/PS18kh reported by Holoien et al. and detected during ZTF commissioning. The ZTF light curve of the tidal disruption event (TDE) samples the rise-to-peak exceptionally well, with 50 days of g- and r-band detections before the time of maximum light. We also present our multi-wavelength follow-up observations, including the detection of a thermal (kT ≈ 100 eV) X-ray source that is two orders of magnitude fainter than the contemporaneous optical/UV blackbody luminosity, and a stringent upper limit to the radio emission. We use observations of 128 known active galactic nuclei (AGNs) to assess the quality of the ZTF astrometry, finding a median host-flare distance of 0farcs2 for genuine nuclear flares. Using ZTF observations of variability from known AGNs and supernovae we show how these sources can be separated from TDEs. A combination of light-curve shape, color, and location in the host galaxy can be used to select a clean TDE sample from multi-band optical surveys such as ZTF or the Large Synoptic Survey Telescope.

Status
Published
Date
February 25th, 2019
Article doi
https://doi.org/10.3847/1538-4357/aafe0c
Discussion Paper arxiv
https://arxiv.org/abs/1809.02608
14. 2900 Square Degree Search for the Optical Counterpart of Short Gamma-Ray Burst GRB 180523B with the Zwicky Transient Facility
Authors
Coughlin, Michael W., Ahumada, Tomás, Cenko, S. B., Cunningham, Virginia, Ghosh, Shaon, Singer, Leo P., Bellm, Eric C., Burns, Eric, De, Kishalay, Goldstein, Adam, Golkhou, V. Zach, Kaplan, David L., Kasliwal, Mansi M., Perley, Daniel A., Sollerman, J., Bagdasaryan, Ashot, Dekany, Richard, Duev, Dmitry A., Feeney, Michael, Graham, Matthew J., Hale, David, Kulkarni, S. R., Kupfer, Thomas, Laher, Russ R., Mahabal, Ashish, Masci, Frank J., Miller, Adam A., Neill, James D., Patterson, Maria T., Riddle, Reed, Rusholme, Ben, Smith, Roger, Tachibana, Yutaro, Walters, Richard

There is significant interest in the models for production of short gamma-ray bursts (GRBs). Until now, the number of known short GRBs with multi-wavelength afterglows has been small. While the Fermi GRB Monitor detects many GRBs relative to the Neil Gehrels Swift Observatory, the large localization regions makes the search for counterparts difficult. With the Zwicky Transient Facility (ZTF) recently achieving first light, it is now fruitful to use its combination of depth (m_(AB) ~ 20.6), field of view (≈47 square degrees), and survey cadence (every ~3 days) to perform Target of Opportunity observations. We demonstrate this capability on GRB 180523B, which was recently announced by the Fermi GRB Monitor as a short GRB. ZTF imaged ≈2900 square degrees of the localization region, resulting in the coverage of 61.6% of the enclosed probability over two nights to a depth of m_(AB) ~ 20.5. We characterized 14 previously unidentified transients, and none were found to be consistent with a short GRB counterpart. This search with the ZTF shows it is an efficient camera for searching for coarsely localized short GRB and gravitational-wave counterparts, allowing for a sensitive search with minimal interruption to its nominal cadence.

Status
Published
Date
February 20th, 2019
Article doi
https://doi.org/10.1088/1538-3873/aaff99
Discussion Paper arxiv
https://arxiv.org/abs/1901.11385
13. The Zwicky Transient Facility: Science Objectives
Authors
Graham, Matthew J., Kulkarni, S. R., Bellm, Eric C., Adams, S. M., Barbarino, Cristina, Blagorodnova, N., Bodewits, Dennis, Bolin, Bryce, Brady, Patrick R., Cenko, S. B., Chang, Chan-Kao, Coughlin, M. W., De, Kishalay, Eadie, Gwendolyn, Farnham, Tony L., Feindt, U., Franckowiak, A., Fremling, C., Gezari, Suvi, Ghosh, Shaon, Goldstein, Daniel A., Golkhou, V. Zach, Goobar, Ariel, Ho, Anna Y. Q., Huppenkothen, Daniela, Ivezić, Željko, Jones, R. Lynne, Jurić, Mario, Kaplan, David L., Kasliwal, Mansi M., Kelley, Michael S. P., Kupfer, T., Lee, Chien-De, Lin, Hsing Wen, Lunnan, Ragnhild, Mahabal, Ashish, Miller, Adam A., Ngeow, Chow-Choong, Nugent, P. E., Ofek, Eran O., Prince, Thomas A., Rauch, L., van Roestel, J., Schulze, S., Singer, Leo P., Sollerman, J., Taddia, F., Yan, Lin, Ye, Quan-Zhi, Yu, Po-Chieh, Barlow, Tom A., Bauer, James, Beck, Ron, Belicki, Justin, Biswas, Rahul, Brinnel, Valery, Brooke, Tim, Bue, Brian, Bulla, Mattia, Buruss, Rick, Connolly, Andrew, Cunningham, Virginia, Dekany, Richard, Delacroix, Alex, Desai, Vandana, Duev, Dmitry A., Feeney, Michael, Flynn, David, Frederick, Sara, Gal-Yam, A., Giomi, Matteo, Groom, Steven L., Hacopians, Eugean, Hale, David, Helou, G., Henning, John, Hover, David, Hillenbrand, Lynne A., Howell, Justin, Hung, Tiara, Imel, David, Ip, Wing-Huen, Jackson, Edward, Kaspi, S., Kaye, Stephen, Kowalski, Marek, Kramer, Emily, Kuhn, Michael A., Landry, Walter, Laher, Russ R., Mao, Peter H., Masci, Frank J., Monkewitz, Serge, Murphy, Patrick, Nordin, Jakob, Patterson, Maria T., Penprase, B. E., Porter, Michael, Rebbapragada, U. D., Reiley, Daniel J., Riddle, Reed, Rigault, M., Rodriguez, Hector, Rusholme, B., van Santen, J., Shupe, D. L., Smith, Roger M., Soumagnac, Maayane T., Stein, Robert, Surace, Jason, Szkody, Paula, Terek, Scott, Van Sistine, Angela, van Velzen, Sjoert, Vestrand, W. T., Walters, Richard, Ward, Charlotte A., Zhang, Chaoran, Zolkower, Jeffry

The Zwicky Transient Facility (ZTF), a public–private enterprise, is a new time-domain survey employing a dedicated camera on the Palomar 48-inch Schmidt telescope with a 47 deg^2 field of view and an 8 second readout time. It is well positioned in the development of time-domain astronomy, offering operations at 10% of the scale and style of the Large Synoptic Survey Telescope (LSST) with a single 1-m class survey telescope. The public surveys will cover the observable northern sky every three nights in g and r filters and the visible Galactic plane every night in g and r. Alerts generated by these surveys are sent in real time to brokers. A consortium of universities that provided funding ("partnership") are undertaking several boutique surveys. The combination of these surveys producing one million alerts per night allows for exploration of transient and variable astrophysical phenomena brighter than r ~ 20.5 on timescales of minutes to years. We describe the primary science objectives driving ZTF, including the physics of supernovae and relativistic explosions, multi-messenger astrophysics, supernova cosmology, active galactic nuclei, and tidal disruption events, stellar variability, and solar system objects.

Status
Published
Date
February 13th, 2019
Article doi
https://doi.org/10.1088/1538-3873/ab006c
Discussion Paper arxiv
https://arxiv.org/abs/1902.01945
12. The GROWTH Marshal: A Dynamic Science Portal for Time-domain Astronomy
Authors
Kasliwal, M. M., Cannella, C., Bagdasaryan, A., Hung, T., Feindt, U., Singer, L. P., Coughlin, M., Fremling, C., Walters, R., Duev, D., Itoh, R., Quimby, R. M.

We describe a dynamic science portal called the GROWTH Marshal that allows time-domain astronomers to define science programs; program filters to save sources from different discovery streams; coordinate follow-up with various robotic or classical telescopes; analyze the panchromatic follow-up data; and generate summary tables for publication. The GROWTH marshal currently serves 137 scientists, 38 science programs, and 67 telescopes. Every night, in real time, several science programs apply various customized filters to the 10^5 nightly alerts from the Zwicky Transient Facility. Here, we describe the schematic and explain the functionality of the various components of this international collaborative platform.

Status
Published
Date
February 7th, 2019
Article doi
https://doi.org/10.1088/1538-3873/aafbc2
Discussion Paper arxiv
https://arxiv.org/abs/1902.01934
11. Machine Learning for the Zwicky Transient Facility
Authors
Mahabal, Ashish, Rebbapragada, U. D., Walters, Richard, Masci, Frank J., Blagorodnova, N., van Roestel, Jan, Biswas, Rahul, Burdge, Kevin B., Chang, Chan-Kao, Duev, Dmitry A., Golkhou, V. Zach, Miller, Adam A., Nordin, Jakob, Ward, Charlotte, Adams, Scott M., Bellm, Eric C., Branton, Doug, Bue, Brian D., Cannella, C., Connolly, Andrew, Dekany, Richard, Feindt, Ulrich, Hung, Tiara, Fortson, Lucy F., Frederick, Sara, Fremling, C., Gezari, Suvi, Graham, Matthew J., Groom, Steven, Kasliwal, Mansi M., Kulkarni, S. R., Kupfer, Thomas, Lin, Hsing Wen, Lintott, Chris J., Lunnan, R., Parejko, John, Prince, Thomas A., Riddle, Reed, Rusholme, B., Saunders, Nicholas, Sedaghat, Nima, Shupe, David L., Singer, Leo P., Soumagnac, Maayane T., Szkody, Paula, Tachibana, Yutaro, Tirumala, Kushal, van Velzen, Sjoert, Wright, Darryl

The Zwicky Transient Facility is a large optical survey in multiple filters producing hundreds of thousands of transient alerts per night. We describe here various machine learning (ML) implementations and plans to make the maximal use of the large data set by taking advantage of the temporal nature of the data, and further combining it with other data sets. We start with the initial steps of separating bogus candidates from real ones, separating stars and galaxies, and go on to the classification of real objects into various classes. Besides the usual methods (e.g., based on features extracted from light curves) we also describe early plans for alternate methods including the use of domain adaptation, and deep learning. In a similar fashion we describe efforts to detect fast moving asteroids. We also describe the use of the Zooniverse platform for helping with classifications through the creation of training samples, and active learning. Finally we mention the synergistic aspects of ZTF and LSST from the ML perspective.

Status
Published
Date
January 31st, 2019
Article doi
https://doi.org/10.1088/1538-3873/aaf3fa
Discussion Paper arxiv
https://arxiv.org/abs/1902.01936
10. Exposure-time Correction for the ZTF Camera
Authors
Giomi, M., Smith, R. M., Kupfer, T., Nordin, J.

A correction factor is derived to account for nonuniformities in the exposure time as seen by different positions on the focal plane of the ZTF camera. These nonuniformities arise from a combination of the shutter opening and closing motion; the geometrical setup of the telescope; and the presence of obstructions in the beam beyond the pupil plane. The correction to the exposure time is measured by comparing a set of dome-flat images taken with multiple shutter opening/closing motions to those obtained with a normal exposure of the same total duration. The correction to the exposure time is small, of the order of 30 ms averaged over the entire field of view with peaks of ~60 ms at the east and west edges of the camera. The effect is thus of the order of 0.1% on average for the nominal ZTF survey exposures of 30 s. Comparing the results obtained from three sets of dedicated observations acquired in 2017 December, we assess the stability of the correction factor to be better than 7% both in time and with respect to variations of the experimental conditions.

Status
Published
Date
January 31st, 2019
Article doi
https://doi.org/10.1088/1538-3873/aaf58e
9. The Zwicky Transient Facility: System Overview, Performance, and First Results
Authors
Bellm, Eric C., Kulkarni, Shrinivas R., Graham, Matthew J., Dekany, Richard, Smith, Roger M., Riddle, Reed, Masci, Frank J., Helou, George, Prince, Thomas A., Adams, Scott M., Barbarino, C., Barlow, Tom A., Bauer, James M., Beck, R., Belicki, Justin, Biswas, Rahul, Blagorodnova, N., Bodewits, Dennis, Bolin, Bryce, Brinnel, V., Brooke, T., Bue, Brian, Bulla, Mattia, Burruss, Rick S., Cenko, S. B., Chang, Chan-Kao, Connolly, Andrew, Coughlin, Michael W., Cromer, John, Cunningham, Virginia, De, Kishalay, Delacroix, Alex, Desai, Vandana, Duev, Dmitry A., Eadie, Gwendolyn, Farnham, Tony L., Feeney, Michael, Feindt, U., Flynn, David, Franckowiak, A., Frederick, Sara, Fremling, C., Gal-Yam, A., Gezari, Suvi, Giomi, Matteo, Goldstein, Daniel A., Golkhou, V. Zach, Goobar, A., Groom, Steven, Hacopians, Eugean, Hale, David, Henning, John, Ho, Anna Y. Q., Hover, David, Howell, Justin, Hung, Tiara, Huppenkothen, Daniela, Imel, D., Ip, Wing-Huen, Ivezić, Željko, Jackson, Edward, Jones, Lynne, Jurić, Mario, Kasliwal, Mansi M., Kaspi, S., Kaye, Stephen, Kelley, Michael S. P., Kowalski, M., Kramer, Emily, Kupfer, T., Landry, Walter, Laher, Russ R., Lee, Chien-De, Lin, Hsing Wen, Lin, Zhong-Yi, Lunnan, Ragnhild, Giomi, Matteo, Mahabal, Ashish, Mao, Peter H., Miller, Adam A., Monkewitz, Serge, Murphy, Patrick, Ngeow, Chow-Choong, Nordin, Jakob, Nugent, P. E., Ofek, Eran O., Patterson, Maria T., Penprase, B. E., Porter, Michael, Rauch, Ludwig, Rebbapragada, U. D., Reiley, Daniel J., Rigault, Michael, Rodriguez, Hector, van Roestel, J., Rusholme, B., van Santen, J., Schulze, S., Shupe, D. L., Singer, Leo P., Soumagnac, Maayane T., Stein, Robert, Surace, Jason, Sollerman, J., Szkody, Paula, Taddia, F., Terek, Scott, Van Sistine, Angela, van Velzen, Sjoert, Vestrand, W. T., Walters, Richard, Ward, Charlotte, Yi, Quan-Zhi, Yu, Po-Chieh, Yan, Lin

The Zwicky Transient Facility (ZTF) is a new optical time-domain survey that uses the Palomar 48 inch Schmidt telescope. A custom-built wide-field camera provides a 47 deg^2 field of view and 8 s readout time, yielding more than an order of magnitude improvement in survey speed relative to its predecessor survey, the Palomar Transient Factory. We describe the design and implementation of the camera and observing system. The ZTF data system at the Infrared Processing and Analysis Center provides near-real-time reduction to identify moving and varying objects. We outline the analysis pipelines, data products, and associated archive. Finally, we present on-sky performance analysis and first scientific results from commissioning and the early survey. ZTF's public alert stream will serve as a useful precursor for that of the Large Synoptic Survey Telescope.

Status
Published
Date
December 7th, 2018
Article doi
https://doi.org/10.1088/1538-3873/aaecbe
8. The Zwicky Transient Facility: Data Processing, Products, and Archive
Authors
Masci, Frank J., Laher, Russ R., Rusholme, B., Shupe, D. L., Groom, Steven, Surace, J., Jackson, Edward, Monkewitz, Serge, Beck, R., Flynn, David, Terek, Scott, Landry, Walter, Hacopians, Eugean, Desai, Vandana, Howell, Justin, Brooke, Timothy Y., Imel, D., Wachter, Stefanie, Ye, Quan-Zhi, Lin, Hsing-Wen, Cenko, S. B., Cunningham, Virginia, Rebbapragada, U. D., Bue, B. D., Miller, Adam A., Mahabal, Ashish, Bellm, Eric C., Patterson, Maria T., Jurić, Mario, Golkhou, V. Zach, Ofek, Eran O., Walters, Richard, Graham, Matthew J., Kasliwal, Mansi M., Dekany, Richard G., Kupfer, T., Burdge, Kevin B., Cannella, C., Barlow, Thomas, Van Sistine, Angela, Giomi, Matteo, Fremling, Christoffer, Blagorodnova, N., Levitan, David, Riddle, Reed, Smith, Roger, Helou, G., Prince, Thomas A., Kulkarni, S. R.

The Zwicky Transient Facility (ZTF) is a new robotic time-domain survey currently in progress using the Palomar 48-inch Schmidt Telescope. ZTF uses a 47 square degree field with a 600 megapixel camera to scan the entire northern visible sky at rates of ~3760 square degrees/hour to median depths of g ~ 20.8 and r ~ 20.6 mag (AB, 5σ in 30 sec). We describe the Science Data System that is housed at IPAC, Caltech. This comprises the data-processing pipelines, alert production system, data archive, and user interfaces for accessing and analyzing the products. The real-time pipeline employs a novel image-differencing algorithm, optimized for the detection of point-source transient events. These events are vetted for reliability using a machine-learned classifier and combined with contextual information to generate data-rich alert packets. The packets become available for distribution typically within 13 minutes (95th percentile) of observation. Detected events are also linked to generate candidate moving-object tracks using a novel algorithm. Objects that move fast enough to streak in the individual exposures are also extracted and vetted. We present some preliminary results of the calibration performance delivered by the real-time pipeline. The reconstructed astrometric accuracy per science image with respect to Gaia DR1 is typically 45 to 85 milliarcsec. This is the RMS per-axis on the sky for sources extracted with photometric S/N ≥ 10 and hence corresponds to the typical astrometric uncertainty down to this limit. The derived photometric precision (repeatability) at bright unsaturated fluxes varies between 8 and 25 millimag. The high end of these ranges corresponds to an airmass approaching ~2—the limit of the public survey. Photometric calibration accuracy with respect to Pan-STARRS1 is generally better than 2%. The products support a broad range of scientific applications: fast and young supernovae; rare flux transients; variable stars; eclipsing binaries; variability from active galactic nuclei; counterparts to gravitational wave sources; a more complete census of Type Ia supernovae; and solar-system objects.

Status
Published
Date
December 7th, 2018
Article doi
https://doi.org/10.1088/1538-3873/aae8ac
7. The Zwicky Transient Facility Alert Distribution System
Authors
Patterson, Maria T., Bellm, Eric C., Rusholme, Ben, Masci, Frank J., Juric, Mario, Krughoff, K. Simon, Golkhou, V. Zach, Graham, Matthew J., Kulkarni, Shrinivas R., Helou, George

The Zwicky Transient Facility (ZTF) survey generates real-time alerts for optical transients, variables, and moving objects discovered in its wide-field survey. We describe the ZTF alert stream distribution and processing (filtering) system. The system uses existing open-source technologies developed in industry: Kafka, a real-time streaming platform, and Avro, a binary serialization format. The technologies used in this system provide a number of advantages for the ZTF use case, including (1) built-in replication, scalability, and stream rewind for the distribution mechanism; (2) structured messages with strictly enforced schemas and dynamic typing for fast parsing; and (3) a Python-based stream processing interface that is similar to batch for a familiar and user-friendly plug-in filter system, all in a modular, primarily containerized system. The production deployment has successfully supported streaming up to 1.2 million alerts or roughly 70 GB of data per night, with each alert available to a consumer within about 10 s of alert candidate production. Data transfer rates of about 80,000 alerts/minute have been observed. In this paper, we discuss this alert distribution and processing system, the design motivations for the technology choices for the framework, performance in production, and how this system may be generally suitable for other alert stream use cases, including the upcoming Large Synoptic Survey Telescope.

Status
Published
Date
November 27th, 2018
Article doi
https://doi.org/10.1088/1538-3873/aae904
Discussion Paper arxiv
https://arxiv.org/abs/1902.02227
6. A Morphological Classification Model to Identify Unresolved PanSTARRS1 Sources: Application in the ZTF Real-time Pipeline
Authors
Tachibana, Yutaro, Miller, A. A.

In the era of large photometric surveys, the importance of automated and accurate classification is rapidly increasing. Specifically, the separation of resolved and unresolved sources in astronomical imaging is a critical initial step for a wide array of studies, ranging from Galactic science to large scale structure and cosmology. Here, we present our method to construct a large, deep catalog of point sources utilizing Pan-STARRS1 (PS1) 3π survey data, which consists of ~3 × 10^9 sources with m ≾ 23.5 mag. We develop a supervised machine-learning methodology, using the random forest (RF) algorithm, to construct the PS1 morphology model. We train the model using ~5 × 10^4 PS1 sources with HST COSMOS morphological classifications and assess its performance using ~4 × 10^6 sources with Sloan Digital Sky Survey (SDSS) spectra and ~2 × 108^ Gaia sources. We construct 11 "white flux" features, which combine PS1 flux and shape measurements across five filters, to increase the signal-to-noise ratio relative to any individual filter. The RF model is compared to three alternative models, including the SDSS and PS1 photometric classification models, and we find that the RF model performs best. By number the PS1 catalog is dominated by faint sources (m ≳ 21 mag), and in this regime the RF model significantly outperforms the SDSS and PS1 models. For time-domain surveys, identifying unresolved sources is crucial for inferring the Galactic or extragalactic origin of new transients. We have classified ~1.5 × 10^9 sources using the RF model, and these results are used within the Zwicky Transient Facility real-time pipeline to automatically reject stellar sources from the extragalactic alert stream.

Status
Published
Date
November 13th, 2018
Article doi
https://doi.org/10.1088/1538-3873/aae3d9
Discussion Paper arxiv
https://arxiv.org/abs/1902.01935
5. Initial performance of the Zwicky transient facility: a wide-fast time-domain survey
Authors
Dekany, Richard G., Smith, Roger, Riddle, Reed, Feeney, Michael, Kaye, Stephen, Porter, Michael, Hale, David, Zolkower, Jeffry, Mao, Peter, Reiley, Daniel, Murphy, Patrick, Rodriguez, Hector, Belicki, Justin, Henning, John, Cromer, John

Zwicky Transient Facility is an integrated, multi-band astronomical survey system optimized for sensitivity, observing cadence, and efficiency. The key subsystem consists of a 600 megapixel CCD focal plane mounted in a flat-fielding vacuum cryostat, located at the prime focus of the 1.2-meter Samuel Oschin Telescope at Palomar Observatory. Supporting subsystems include a new 2.4-meter optical shutter assembly, a 1.35-meter diameter aspheric corrector plate, a cryostat stabilizing hexapod, a commercial robotic arm-based exchanger, three 440 millimeter width filters, four guide/focus CCDs, and dedicated optics compensating individual field curvature over each of sixteen 6k x 6k science CCDs.To optimize ZTF efficiency, all telescope and dome drives were upgraded for higher speed and acceleration, fast readout electronics were implemented, and a sophisticated robotic control system has been implemented. We present for the first time on-sky results from the recently completed ZTF including its realized optical image quality, CCD noise, and observing efficiency performance and discuss engineering challenges that have been overcome. Early scientific results from the ZTF survey are also included.

Status
Published
Date
July 17th, 2018
Article doi
https://doi.org/10.1117/12.2312353
4. The Zwicky transient facility robotic observing system
Authors
Riddle, Reed, Cromer, John, Hale, David, Henning, John, Baker, John, Milburn, Jennifer, Kaye, Stephen, Bellm, Eric C., Walters, Richard, Dekany, Richard, Smith, Roger

The transient universe is fast becoming one of the most important research areas in astronomy. Finding objects that change, either quickly or periodically, has opened up new understanding of the cosmos around us, and brought up new questions that require further investigation. The Zwicky Transient Facility (ZTF) has been developed to observe as much of the sky as possible at a rapid rate, in order to expand the regime of time domain measurement to shorter intervals and detect changes in the sky more quickly. ZTF is a fully automated system, composed of the Samuel Oschin 48-inch (1.2m) telescope at Palomar Observatory (P48), the mosaic camera constructed by Caltech, a filter exchange system, associated sensors and electrical systems, and the Robotic Observing Software (ROS) that controls the operation of the entire system. P48 is a 70 year old telescope that has been upgraded with new hardware, electronics, and a modern telescope control system to allow it to move quickly and accurately across the sky under robotic control. The ZTF mosaic camera is a custom system composed of 16 6Kx6K pixel CCDs, creating a mosaic camera with over 576 million pixels that can image 47 square degrees down to a magnitude of 20.5 in a 30 second exposure. The filter exchange system uses a Kuka robotic arm to grab the 400x450mm filters out of a storage closet and place them onto the front of the mosaic camera, where they are held in place by electromagnets and locking pins. A full sensor system monitors the health of the camera dewar and environment of the observatory; a separate weather station monitors the outside environment. Other subsystems control the motion of the Hexapod that the mosaic camera is mounted on, the top end shutter, and remote switching of power, Managing all of these subsystems is ROS, which is the automated control software that runs ZTF observations. ROS is based on the Robo-AO control system, with improved automation procedures and expanded capabilities to handle the operations required for ZTF. ROS consists of 31 separate software daemons spread across 5 computer systems (4 to control the mosaic camera, 1 for robotic operation); the robotic control daemon is able to manage all daemons, as well as start and stop their operation as necessary. Watchdog daemons intervene in case of robotic system problems, and each daemon has an internal watchdog that can fix or kill the daemon in case of difficulties; if a daemon dies the robotic system automatically restarts it. ROS controls the start of observations and morning shut down, handles weather monitoring and safely stopping in case of bad weather, and responds to problems in the observing sequence by fixing them or stopping operations and sending a message for help. All calibration measurements are done automatically at the beginning of the night; if the calibrations are interrupted they are completed after observations finish in the morning. A queue system determines the observation priority and revises the order of observations dynamically to optimize observational efficiency. ROS is able to operate with less than 15 second overhead between each standard ZTF observation (with a 7.5 degree slew); this is achieved by reading out thee mosaic camera during telescope slew, then transferring and writing FITS data files during the next exposure. FITS headers are kept synchronized through daemons that gather all relevant FITS header information and distribute that to the camera computers. ROS is able to produce more than 80 mosaic science images per hour in standard survey mode; each mosaic is a total of 380MB compressed, so the system produces more than 30GB of data on disk per hour that have to be transferred off the mountain. A new data transfer system synchronizes the compressed FITS data files to the data analysis servers in Pasadena, CA in parallel with the observing system; images are in place for the data analysis pipelines in less than a minute after the ZTF shutter closes. This presentation will discuss the development and execution of the ZTF observing software, as well as analyze the observational behavior and efficiency of the system during the first few months of on-sky science observations.

Status
Published
Date
July 17th, 2018
Article doi
https://doi.org/10.1117/12.2312702
3. Optical design of the Zwicky Transient Facility: a major upgrade to the 48” Schmidt Camera at Palomar Observatory
Authors
Reiley, Daniel J., Dekany, Richard G., Smith, Roger M., Delacroix, Alexandre, Feeney, Michael, Callahan, Shawn

The Zwicky Transient Facility (ZTF) will be a major upgrade to the 48” Schmidt Camera at Palomar Observatory, which was initially commissioned in 1948. Although the optical design for ZTF is a relatively small part of the project, system requirements placed special constraints on the optical design. This paper presents the optical design for ZTF as well as the system requirements that drove the optical design.

2. The Zwicky Transient Facility Camera
Authors
Dekany, Richard, Smith, Roger M., Belicki, Justin, Delacroix, Alexandre, Duggan, Gina, Feeney, Michael, Hale, David, Kaye, Stephen, Milburn, Jennifer, Murphy, Patrick, Porter, Michael, Reiley, Dan, Riddle, Reed, Rodriguez, Hector, Bellm, Eric

The Zwicky Transient Facility Camera (ZTFC) is a key element of the ZTF Observing System, the integrated system of optoelectromechanical instrumentation tasked to acquire the wide-field, high-cadence time-domain astronomical data at the heart of the Zwicky Transient Facility. The ZTFC consists of a compact cryostat with large vacuum window protecting a mosaic of 16 large, wafer-scale science CCDs and 4 smaller guide/focus CCDs, a sophisticated vacuum interface board which carries data as electrical signals out of the cryostat, an electromechanical window frame for securing externally inserted optical filter selections, and associated cryo-thermal/vacuum system support elements. The ZTFC provides an instantaneous 47 deg^2 field of view, limited by primary mirror vignetting in its Schmidt telescope prime focus configuration. We report here on the design and performance of the ZTF CCD camera cryostat and report results from extensive Joule-Thompson cryocooler tests that may be of broad interest to the instrumentation community.

1. The Zwicky Transient Facility Observing System
Authors
Smith, Roger M., Dekany, Richard G., Bebek, Christopher, Bellm, Eric, Bui, Khanh, Cromer, John, Gardner, Paul, Hoff, Matthew, Kaye, Stephen, Kulkarni, Shrinivas, Lambert, Andrew, Levi, Michael, Reiley, Dan

The Zwicky Transient Facility (ZTF) is a synoptic optical survey for high-cadence time-domain astronomy. Building upon the experience and infrastructure of the highly successful Palomar Transient Factory (PTF) team, ZTF will survey more than an order of magnitude faster than PTF in sky area and volume in order to identify rare, rapidly varying optical sources. These sources will include a trove of supernovae, exotic explosive transients, unusual stellar variables, compact binaries, active galactic nuclei, and asteroids. The single-visit depth of 20.4 mag is well matched to spectroscopic follow-up observations, while the co-added images will provide wide sky coverage 1.5 – 2 mag deeper than SDSS. The ZTF survey will cover the entire Northern Sky and revisit fields on timescales of a few hours, providing hundreds of visits per field each year, an unprecedented cadence, as required to detect fast transients and variability. This high-cadence survey is enabled by an observing system based on a new camera having 47 deg^2 field of view – a factor of 6.5 greater than the existing PTF camera - equipped with fast readout electronics, a large, fast exposure shutter, faster telescope and dome drives, and various measures to optimize delivered image quality. Our project has already received an initial procurement of e2v wafer-scale CCDs and we are currently fabricating the camera cryostat. International partners and the NSF committed funds in June 2014 so construction can proceed as planned to commence engineering commissioning in 2016 and begin operations in 2017. Public release will allow broad utilization of these data by the US astronomical community. ZTF will also promote the development of transient and variable science methods in preparation for the seminal first light of LSST.