Measurement of the rate of stellar tidal disruption flares
  We report an observational estimate of the rate of stellar tidal disruption
flares (TDFs) in inactive galaxies, based on a successful search for these
events among transients in galaxies using archival SDSS multi-epoch imaging
data (Stripe 82). This search yielded 186 nuclear flares in galaxies, of which
two are excellent TDF candidates. Because of the systematic nature of the
search, the very large number of galaxies, the long time of observation, and
the fact that non-TDFs were excluded without resorting to assumptions about TDF
characteristics, this study provides an unparalleled opportunity to measure the
TDF rate. To compute the rate of optical stellar tidal disruption events, we
simulate our entire pipeline to obtain the efficiency of detection. The rate
depends on the light curves of TDFs, which are presently still poorly
constrained. Using only the observed part of the SDSS light curves gives a
model-independent upper limit to the optical TDF rate: < 2 10^-4 per year per
galaxy (90% CL). We develop three empirical models of the light curves, based
on the two SDSS light curves and two more recent and better-sampled Pan-STARRS
TDF light curves, leading to our best-estimate of the rate: (1.5 -
2.0)_{-1.3}^{+2.7} 10^-5 per year per galaxy. We explore the modeling
uncertainties by considering two theoretically motivated light curve models, as
well as two different relationships between black hole mass and galaxy
luminosity, and two different treatments of the cutoff in the visibility of
TDFs at large black hole mass. From this we conclude that these sources of
uncertainty are not significantly larger than the statistical ones. Our results
are applicable for galaxies hosting black holes with mass in the range of few
million to 10^8 solar masses, and translates to a volumetric TDF rate of (4 -
8) 10^-8 per year per cubic Mpc.
