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 (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.
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 identified
, of which only one
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.
Compact Galactic binaries where at least one member is a white dwarf (WD) or neutron star constitute the majority of individually detectable sources for future low-frequency space-based ...gravitational-wave (GW) observatories; they also form an unresolved continuum, the dominant Galactic foreground at frequencies below a few mHz. Due to the paucity of electromagnetic observations, the majority of studies of Galactic-binary populations so far have been based on population-synthesis simulations. However, recent surveys have reported several new detections of WD binaries, providing new constraints for population estimates. In this article, we evaluate the impact of revised local densities of interacting WD binaries on future GW observations. Specifically, we consider five scenarios that explain these densities with different assumptions on the formation of interacting systems; we simulate corresponding populations of detached and interacting WD binaries; we estimate the number of individually detectable GW sources and the magnitude of the confusion-noise foreground, as observed by space-based detectors with 5 and 1 Mkm arms. We confirm earlier estimates of thousands of detached-binary detections, but project only a few ten to a few hundred detections of interacting systems. This reduction is partly due to our assessment of detection prospects, based on the iterative identification and subtraction of bright sources with respect to both instrument and confusion noise. We also confirm earlier estimates for the confusion-noise foreground, except in one scenario that explains smaller local densities of interacting systems with smaller numbers of progenitor detached systems.
Most massive stars end their lives as red supergiants (RSGs), a short-lived evolutionary phase when they are known to pulsate with varying amplitudes. The RSG period-luminosity (PL) relation has been ...measured in the Milky Way, the Magellanic Clouds and M33 for about 120 stars in total. Using over 1500 epochs of R-band monitoring from the Palomar Transient Factory survey over a five-year period, we study the variability of 255 spectroscopically cataloged RSGs in M31. We find that all RGSs brighter than MK −10 mag (log(L/L ) > 4.8) are variable at ΔmR > 0.05 mag. Our period analysis finds 63 with significant pulsation periods. Using the periods found and the known values of MK for these stars, we derive the RSG PL relation in M31 and show that it is consistent with those derived earlier in other galaxies of different metallicities. We also detect, for the first time, a sequence of likely first-overtone pulsations. Comparison to stellar evolution models from MESA confirms the first-overtone hypothesis and indicates that the variable stars in this sample have 12 M < M < 24 M . As these RSGs are the immediate progenitors to Type II-P core-collapse supernovae (SNe), we also explore the implication of their variability in the initial-mass estimates for SN progenitors based on archival images of the progenitors. We find that this effect is small compared to the present measurement errors.
Spectroscopic detection of narrow emission lines traces the presence of circumstellar mass distributions around massive stars exploding as core-collapse supernovae. Transient emission lines ...disappearing shortly after the supernova explosion suggest that the material spatial extent is compact and implies an increased mass loss shortly prior to explosion. Here, we present a systematic survey for such transient emission lines (Flash Spectroscopy) among Type II supernovae detected in the first year of the Zwicky Transient Facility survey. We find that at least six out of ten events for which a spectrum was obtained within two days of the estimated explosion time show evidence for such transient flash lines. Our measured flash event fraction (>30% at 95% confidence level) indicates that elevated mass loss is a common process occurring in massive stars that are about to explode as supernovae.
Abstract
Using photometry collected with the Zwicky Transient Facility, we are conducting an ongoing survey for binary systems with short orbital periods (
with the goal of identifying new ...gravitational-wave sources detectable by the upcoming Laser Interferometer Space Antenna (LISA). We present a sample of 15 binary systems discovered thus far, with orbital periods ranging from 6.91 to 56.35 minutes. Of the 15 systems, seven are eclipsing systems that do not show signs of significant mass transfer. Additionally, we have discovered two AM Canum Venaticorum systems and six systems exhibiting primarily ellipsoidal variations in their lightcurves. We present follow-up spectroscopy and high-speed photometry confirming the nature of these systems, estimates of their LISA signal-to-noise ratios, and a discussion of their physical characteristics.
Using the Zwicky Transient Facility alert stream, we are conducting a large spectroscopic campaign to construct a complete, volume-limited sample of transients brighter than 20 mag, and coincident ...within 100″ of galaxies in the Census of the Local Universe catalog. We describe the experiment design and spectroscopic completeness from the first 16 months of operations, which have classified 754 supernovae. We present results from a systematic search for calcium-rich gap transients in the sample of 22 low-luminosity (peak absolute magnitude M > −17), hydrogen-poor events found in the experiment. We report the detection of eight new events, and constrain their volumetric rate to 15% 5% of the SN Ia rate. Combining this sample with 10 previously known events, we find a likely continuum of spectroscopic properties ranging from events with SN Ia-like features (Ca-Ia objects) to those with SN Ib/c-like features (Ca-Ib/c objects) at peak light. Within the Ca-Ib/c events, we find two populations distinguished by their red (g − r 1.5 mag) or green ( mag) colors at the r-band peak, wherein redder events show strong line blanketing features and slower light curves (similar to Ca-Ia objects), weaker He lines, and lower Ca ii/O i in the nebular phase. We find that all together the spectroscopic continuum, volumetric rates, and striking old environments are consistent with the explosive burning of He shells on low-mass white dwarfs. We suggest that Ca-Ia and red Ca-Ib/c objects arise from the double detonation of He shells, while green Ca-Ib/c objects are consistent with low-efficiency burning scenarios like detonations in low-density shells or deflagrations.
Machine Learning for the Zwicky Transient Facility Mahabal, Ashish; Rebbapragada, Umaa; Walters, Richard ...
Publications of the Astronomical Society of the Pacific,
03/2019, Volume:
131, Issue:
997
Journal Article
Peer reviewed
Open access
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.
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 = 42,400 300 K and a surface gravity of = 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 MsdOB = 0.337 0.015 and a white dwarf accretor with a mass MWD = 0.545 0.020 . Theoretical binary modeling indicates the hot subdwarf formed during a common envelope phase when a 2.5-2.8 star lost its envelope when crossing the Hertzsprung gap. To match its current , , , and masses, we estimate a post-common envelope period of 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 , merge with its carbon/oxygen white dwarf companion after 17 Myr, and presumably explode as a thermonuclear supernova or form an R CrB star.
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