The first tidal disruption flare in ZTF: from photometric selection to multi-wavelength characterization

We report the independent discovery of the transient source AT2018zr during commission observations of the Zwicky Transient Facility (ZTF), the first tidal disruption event(TDE) found in this survey. The ZTF light curveof the 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 that were triggered by the discovery of this flare: the detection of a weak X-ray source ($\nu L_\nu \sim 10^{41}$ erg/s) and a stringent upper limit to the radio emission. The X-ray emission shows a thermal spectrum ($kT \approx 100$ eV) and is two orders of magnitude fainter than the contemporaneous optical/UV blackbody luminosity. We use observations of 128 known active galactic nuclei (AGN) to assess the quality of the ZTF astrometry, finding a median host-flare distance of 0.2" for genuine nuclear flares. Using ZTF observations of variability from known AGN and SNe we show how these sources can be separated from TDEs. A combination of light curveshape, 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 LSST.