Time-domain surveys can exchange sky coverage for revisit frequency, complicating the comparison of their relative capabilities. By using different revisit intervals, a specific camera may execute ...surveys optimized for discovery of different classes of transient objects. We propose a new figure of merit, the instantaneous volumetric survey speed, for evaluating transient surveys. This metric defines the trade between cadence interval and snapshot survey volume and so provides a natural means of comparing survey capability. The related metric of areal survey speed imposes a constraint on the range of possible revisit times: we show that many modern time-domain surveys are limited by the amount of fresh sky available each night. We introduce the concept of "spectroscopic accessibility" and discuss its importance for transient science goals requiring followup observing. We present an extension of the control time algorithm for cases where multiple consecutive detections are required. Finally, we explore how survey speed and choice of cadence interval determine the detection rate of transients in the peak absolute magnitude-decay timescale phase space.
The Zwicky Transient Facility: Surveys and Scheduler Bellm, Eric C.; Kulkarni, Shrinivas R.; Barlow, Tom ...
Publications of the Astronomical Society of the Pacific,
06/2019, Volume:
131, Issue:
1000
Journal Article
Peer reviewed
Open access
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.
The Zwicky Transient Facility: Science Objectives Graham, Matthew J.; Barbarino, Cristina; Blagorodnova, Nadejda ...
Publications of the Astronomical Society of the Pacific,
07/2019, Volume:
131, Issue:
1001
Journal Article
Peer reviewed
Open access
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 deg2 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.
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 deg2 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.
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.
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, 5 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.
Colors of Type Ia supernovae (SNe Ia) in the first few days after explosion provide a potential discriminant between different models. In this paper, we present g − r colors of 65 SNe Ia discovered ...within 5 days from first light by the Zwicky Transient Facility in 2018, a sample that is about three times larger than that in the literature. We find that g − r colors are intrinsically rather homogeneous at early phases, with about half of the dispersion attributable to photometric uncertainties ( 0.18 mag). Colors are nearly constant starting from 6 days after first light (g − r ∼ −0.15 mag), while the time evolution at earlier epochs is characterized by a continuous range of slopes, from events rapidly transitioning from redder to bluer colors (slope of ∼−0.25 mag day−1) to events with a flatter evolution. The continuum in the slope distribution is in good agreement both with models requiring some amount of 56Ni mixed in the outermost regions of the ejecta and with "double-detonation" models having thin helium layers ( ) and varying carbon-oxygen core masses. At the same time, six events show evidence for a distinctive "red bump" signature predicted by double-detonation models with larger helium masses. We finally identify a significant correlation between the early-time g − r slopes and supernova brightness, with brighter events associated to flatter color evolution (p-value = 0.006). The distribution of slopes, however, is consistent with being drawn from a single population, with no evidence for two components as claimed in the literature based on B − V colors.
White dwarfs represent the last stage of evolution of stars with mass less than about eight times that of the Sun and, like other stars, are often found in binaries1,2. If the orbital period ofthe ...binary is short enough, energy losses from gravitational-wave radiation can shrink the orbit until the two white dwarfs come into contact and merge3. Depending on the component masses, the merger can lead to a supernova of type Ia or result in a massive white dwarf4. In the latter case, the white dwarf remnant is expected to be highly magnetized5,6 because ofthe strong magnetic dynamo that should arise during the merger, and be rapidly spinning from the conservation of the orbital angular momentum7. Here we report observations of a white dwarf, ZTF J190132.9+145808.7, that exhibits these properties, but to an extreme: a rotation period of 6.94 minutes, a magnetic field ranging between 600 megagauss and 900 megagauss over its surface, and a stellar radius of 2140+160-230 kilometres, only slightly larger than the radius of the Moon. Such a small radius implies that the star's mass is close to the maximum white dwarf mass, or Chandrasekhar mass. ZTF J190132.9+145808.7 is likely to be cooling through the Urca processes (neutrino emission from electron capture on sodium) because of the high densities reached in its core.
Abstract
Tidal disruption events (TDEs) offer a unique way to study dormant black holes. While the number of observed TDEs has grown thanks to the emergence of wide-field surveys in the past few ...decades, questions regarding the nature of the observed optical, UV, and X-ray emission remain. We present a uniformly selected sample of 30 spectroscopically classified TDEs from the Zwicky Transient Facility Phase I survey operations with follow-up Swift UV and X-ray observations. Through our investigation into correlations between light-curve properties, we recover a shallow positive correlation between the peak bolometric luminosity and decay timescales. We introduce a new spectroscopic class of TDE, TDE-featureless, which are characterized by featureless optical spectra. The new TDE-featureless class shows larger peak bolometric luminosities, peak blackbody temperatures, and peak blackbody radii. We examine the differences between the X-ray bright and X-ray faint populations of TDEs in this sample, finding that X-ray bright TDEs show higher peak blackbody luminosities than the X-ray faint subsample. This sample of optically selected TDEs is the largest sample of TDEs from a single survey yet, and the systematic discovery, classification, and follow-up of this sample allows for robust characterization of TDE properties, an important stepping stone looking forward toward the Rubin era.
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 toward this promise. Here we 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, lower optical luminosities, and higher disruption rates compared to the rest of the sample. 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. A high detection rate of Bowen lines in TDEs with small photometric radii could be explained by the high density that is required for this fluorescence mechanism. The stellar debris can provide a source for this dense material. Diffusion of photons through this debris may explain why the rise and fade timescale of the TDEs in our sample are not correlated. We also report, for the first time, the detection of soft X-ray flares from a TDE on ∼day timescales. Based on the fact that the X-ray 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.
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