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