Ipac logo

Publications

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. 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 in the first few days after explosion provide a potential discriminant between different models. In this paper, we present g−r colors of 38 Type Ia supernovae discovered within 3 days from first light by the Zwicky Transient Facility in 2018, a sample which is twice as large as that in the literature. We find that g−r colors are intrinsically rather homogeneous at early phases, with about half of the dispersion in the first 5 days attributable to photometric uncertainties (σ_(noise)∼σ_(int)∼0.25 mag). Colors are nearly constant starting from 5 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)=0.01M⊙) and varying carbon-oxygen core masses. At the same time, four 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=0.001). 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
Unpublished
Date
April 14th, 2020
Discussion Paper arxiv
http://arxiv.org/abs/2001.00587
64. 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
63. 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.

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 unique, and large, data set of 127 SNe Ia with exquisite coverage by the Zwicky Transient Facility (ZTF). High-cadence (6 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 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 18.5d (with a range extending from ∼15−22d), 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 21.0d (with a range from ∼18−23d). The sample includes a handful or rare and peculiar SNe Ia. Finally, we conclude with a discussion of lessons learned from the ZTF sample that can eventually be applied to Large Synoptic Survey Telescope observations.

Status
Unpublished
Date
April 13th, 2020
Discussion Paper arxiv
http://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 Ultra-Violet 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., Sipocz, 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) in the Ultra-Violet (UV) and visible light. 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 circumstellar material (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% on 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 modelling of these events.

Status
Unpublished
Date
March 10th, 2020
Discussion Paper arxiv
http://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
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. 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, Maayane T., Ofek, E. O., Adams, S. M., Barbarino, C., Bellm, E. C., De, K., Fransson, C., Frederick, S., Golkhou, V. Zach, Graham, Matthew J., Hallakoun, N., Ho, A. Y. Q., Kasliwal, Mansi M., Kaspi, S., Kulkarni, S. R., Laher, R. R., Masci, Frank J., Pozo Nunez, F., 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 two-month science commissioning and early operations of the Zwicky Transient Facility (ZTF) survey. Three of these objects, SN2018bym (ZTF18aapgrxo), SN2018avk (ZTF18aaisyyp) and SN2018bgv (ZTF18aavrmcg) resemble typical SLSN-I spectroscopically, while SN2018don (ZTF18aajqcue) may be an object similar to SN2007bi 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. SN2018bgv 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⊙. SN2018don 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 post-peak seen in a 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
Unpublished
Date
November 21st, 2019
Discussion Paper arxiv
https://arxiv.org/abs/1910.02968
51. 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
50. 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
49. 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
48. 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, Lin, Zhong-Yi, Morbidelli, Alessandro, Bauer, James, Burdge, Kevin B., Coughlin, Michael, Fremling, Christoffer, Itoh, Ryosuke, Koss, Michael, Masci, Frank J., Maeno, Syota, Mamajek, Eric E., Marocco, Federico, Murata, Katsuhiro, Sitko, Michael L., Stern, Daniel, 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, Juric, Mario, Kramer, Emily A., Kulkarni, Shrinivas R., Kupfer, Thomas, Mahabal, Ashish, Neill, James D., Ngeow, Chow-Choong, Penprase, Bryan, Riddle, Reed, Rodriguez, Hector, Rosnet, Philippe, Sollerman, Jesper, Soumagnac, Maayane T.

We present visible and near-infrared photometric and spectroscopic observations of interstellar comet 2I/Borisov taken from 2019 September 10 to 2019 November 03 using the GROWTH collaboration, the Apache Point Observatory ARC 3.5 m and the NASA/IRTF 3.0 m combined with post and pre-discovery 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 (Kelly et al. 2013) shows an object very similar to mildly active Solar System comets with an out-gassing rate of ∼10²⁷ mol/sec. The photometry, taken in filters spanning the visible and NIR range shows a gradual brightening trend of ∼0.03 mags/day since 2019 September 10 UTC for a reddish object becoming neutral in the NIR. The lightcurve from recent and pre-discovery data (Ye et al. 2019) 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 04, we estimate a diameter of 2I's nucleus of ≲3 km, though the true size is likely ∼2-3 times smaller due to the incomplete removal of dust from the measurement. We use the size estimates of 1I/'Oumuamua and 2I/Borisov to roughly estimate the slope of the interstellar object cumulative size-distribution resulting in a slope of ≳-2.9, similar to Solar System comets (Fernandez et al. 2013), though the true slope is likely significantly steeper due to small number statistics and our probable overestimation of the size of 2I.

Status
Unpublished
Date
November 20th, 2019
Discussion Paper arxiv
http://arxiv.org/abs/1910.14004
47. 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
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. SN2018fif: The explosion of a large red supergiant discovered in its infancy by the Zwicky Transient Facility
Authors
Soumagnac, Maayane T., Ganot, Noam, Gal-Yam, Avishay, Ofek, Eran O., Yaron, Ofer, Waxman, Eli, 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, Irani, Ido, Kasliwal, Mansi M., Konidaris, Nicholas P., Kulkarni, Shrinivas R., Kupfer, Thomas, Laher, Russ R., Masci, Frank J., Morag, Jonathan, 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 before. Applying analytical models to such early emission, we can constrain the progenitor stars properties. In this paper, we present observations of SN2018fif (ZTF18abokyfk). 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 SN2018fif was surrounded by relatively small amounts of circumstellar material (CSM) 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 (2017). Compared with previous implementations, SOPRANOS has the advantage of including a careful account of the limited temporal validity domain of the shock-cooling model. We find that the progenitor of SN2018fif was a large red supergiant, with a radius of R=1174_{-81}^{+208} solar radii, and an ejected mass of M=5.6_{-1.0}^{+9.1} solar masses. Our model also gives information on the explosion epoch, the progenitor inner structure, the shock velocity and the extinction. The large radius differs from previously modeled objects, and the difference could be either intrinsic or due to the relatively small amount of CSM around SN2018fif, perhaps making it a "cleaner" candidate for applying shock-cooling analytical models.

Status
Submitted
Date
August 2nd, 2019
Discussion Paper arxiv
http://arxiv.org/abs/1907.11252
36. 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
35. 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
34. 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
33. 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
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, 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.