Optical discovery of probable stellar tidal disruption flares
  Using archival SDSS multi-epoch imaging data (Stripe 82), we have searched
for the tidal disruption of stars by super-massive black holes in non-active
galaxies. Two candidate tidal disruption events (TDEs) are identified. They
have optical black-body temperatures 2 10^4 K and observed peak luminosities
M_g=-18.3 and -20.4; their cooling rates are very low, qualitatively consistent
with expectations for tidal disruption flares. Their properties are examined
using i) SDSS imaging to compare them to other flares observed in the search,
ii) UV emission measured by GALEX and iii) spectra of the hosts and of one of
the flares. Our pipeline excludes optically identifiable AGN hosts, and our
variability monitoring over 9 years provides strong evidence that these are not
flares in hidden AGNs. The spectra and color evolution of the flares are unlike
any SN observed to date, their strong late-time UV emission is particularly
distinctive, and they are nuclear at high resolution, arguing against their
being first cases of a previously-unobserved class of SNe or more extreme
examples of known SN types. Taken together, the observed properties are
difficult to reconcile with a SN or AGN-flare explanation, although an entirely
new process specific to the inner few-hundred parsecs of non-active galaxies
cannot be excluded. Our observed rate and method show the feasibility of
obtaining a candidate TDE sample of hundreds of events and O(1) purity, using
geometric resolution and host and flare color alone. A by-product of this work
is quantification of the power-spectrum of extreme flares in AGNs.
