Luminous Thermal Flares from Quiescent Supermassive Black Holes
  A dormant supermassive black hole lurking in the center of a galaxy will be
revealed when a star passes close enough to be torn apart by tidal forces, and
a flare of electromagnetic radiation is emitted when the bound fraction of the
stellar debris falls back onto the black hole and is accreted. Here we present
the third candidate tidal disruption event discovered in the GALEX Deep Imaging
Survey: a 1.6x10^{43} erg s^{-1} UV/optical flare from a star-forming galaxy at
z=0.1855. The UV/optical SED during the peak of the flare measured by GALEX and
Palomar LFC imaging can be modeled as a single temperature blackbody with
T_{bb}=1.7x10^{5} K and a bolometric luminosity of 3x10^{45} erg s^{-1},
assuming an internal extinction with E(B-V)_{gas}=0.3. The Chandra upper limit
on the X-ray luminosity during the peak of the flare, L_{X}(2-10 keV)< 10^{41}
erg s^{-1}, is 2 orders of magnitude fainter than expected from the ratios of
UV to X-ray flux density observed in active galaxies. We compare the light
curves and broadband properties of all three tidal disruption candidates
discovered by GALEX, and find that (1) the light curves are well fitted by the
power-law decline expected for the fallback of debris from a tidally disrupted
solar-type star, and (2) the UV/optical SEDs can be attributed to thermal
emission from an envelope of debris located at roughly 10 times the tidal
disruption radius of a ~10^{7} M_sun central black hole. We use the observed
peak absolute optical magnitudes of the flares (-17.5 > M_{g} > -18.9) to
predict the detection capabilities of upcoming optical synoptic surveys.
(Abridged)
