Seventeen Tidal Disruption Events from the First Half of ZTF Survey
  Observations: Entering a New Era of Population Studies
  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 towards this
promise. Here 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, as well as
longer rise times and higher disruption rates compared to the rest of the
sample. The Bowen fluorescence mechanism requires a high density which can be
reached at smaller radii, which in turn yields longer diffusion timescales.
Thus, the difference in rise times suggests the pre-peak TDE light curves are
governed not by the fallback timescale, but instead by the diffusion of photons
through the tidal debris. 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. We also report, for the first time, the
detection of soft X-ray flares from a TDE on day timescales. Based on the fact
the 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.
