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.

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;

(Fig. 1). The first BB temperature estimate was obtained via careful extrapolation of the UVOT UV M2 and P60 g + i light curves back to the first P48 (detection) point (see inset of Fig. 1 and text for details). The earlytime BB temperature estimates, within the first half day after explosion, are also in agreement with our temperature estimates from the modeling of the early Keck spectra (Fig. 4), showing the highly ionised emission lines at temperatures > ∼ 50 kK. The shaded region and the solid black line (a running mean of the region) denote bolometric luminosity estimates based on the multiband photometry ( Fig. 1) according to three methods used to calculate the total flux from the SED: interpolation, order-4 polynomial fit, and BB fits. The top end of the shaded region can be regarded as our best lower limit on the real bolometric luminosity, based on the photometric observations. The red triangles denote a (more conservative) lower limit on the bolometric luminosity obtained from our spectra (Fig. 2, Fig. 3), beginning with the early set of 4 Keck spectra at < ∼ 10 hr after explosion and ending with our latest spectrum at 57.2days. The blue triangles show the luminosity as obtained by our best BB temperature and radius estimates (Fig. 4), L = 4πR 2 σT 4 ; the luminosity in the first point, at ∼ 3.8 hr after explosion, exceeds 10 44 erg s −1 .

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.

'Discovery'). No activity had been detected at the same location in the images taken on the previous night and earlier, indicating that the SN likely exploded between May 2.29 and 3.29. Our follow-up spectroscopic campaign (See Extended Data Table 1 for the observation log) established that iPTF14atg was a Type Ia supernova (SN Ia). 3Upon discovery we triggered observations with the Ultraviolet/Optical Telescope (UVOT) and the X-ray Telescope (XRT) onboard the Swift space observatory 11 (observation and data reduction is detailed in Methods subsection 'Data acquisition'; raw measurements are shown in Extended Data Table 2). As can be seen in Figure 1, the UV brightness of iPTF14atg declined substantially in the first two observations. A rough energy flux measure in the UV band is provided by ν f ν ≈ 3×10 −13 ergs cm −2 s −1 in the uvm2 band. Starting from the third epoch, the UV and optical emission began to rise again in a manner similar to that seen in other SNe Ia. The XRT did not detect any X-ray signal at any epoch (Methods subsection 'Data acquisition'). We thus conclude that iPTF 14atg emitted a pulse of radiation primarily in the UV band. This pulse with an observed luminosity of L UV ≈ 3×10 41 ergs s −1 was probably already declining by the first epoch of the Swift observations (within four days of its explosion).Figure 1 also illustrates that such an early UV pulse from a SN Ia within four days of its explosion is unprecedented 12,13 . We now seek an explanation for this early UV emission.As detailed in Methods subsection 'Spherical models for the early UV pulse', we explored models in which the UV emission is spherically symmetric with the SN explosion (such as shock cooling and circumstellar interaction). These models are unable to explain the observed UV pulse. Therefore we turn to asymmetric models in which the UV emission comes from particular directions.A reasonable physical model is UV emission arising in the ejecta as the ejecta encounters a companion 9,14 . When the rapidly moving ejecta slams into the companion, a strong 4 reverse shock is generated in the ejecta that heats up the surrounding material. Thermal radiation from the hot material, which peaks in the ultraviolet, can then be seen for a few days until the fast-moving ejecta engulfs the companion and hides the reverse shock region. We compare a semi-analytical model 9 to the Swift/UVOT lightcurves. For simplicity, we fix the explosion date at May 3. We assume that the exploding white dwarf is close to the Chandrasekhar mass limit (1.4 solar mass) and that the SN explosion energy is 10 51 ergs. These values lead to a mean expansion velocity of 10 4 km s −1 for the ejecta. Since the temperature at the collision location is so high that most atoms are ionized, the opacity is probably dominated by electron scattering. To further simplify the case, we assume that the emission from the reverse shock region is blackbody and isotropic. In order to explain the UV lightcurves, the companion star should be located 60 solar radii away from the w...