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. Although the tidal disruption of a star is a rare event in a galaxy, ≈10^-4 yr^-1, observational candidates have emerged in all-sky X-ray and deep ultraviolet (UV) surveys in the form of luminous UV/X-ray flares from otherwise quiescent galaxies. Here we present the third candidate tidal disruption event discovered in the Galaxy Evolution Explorer (GALEX) Deep Imaging Survey: a 1.6 × 10⁴³ erg s^-1 UV/optical flare from a star-forming galaxy at z = 0.1855. The UV/optical spectral energy distribution (SED) during the peak of the flare measured by GALEX and Palomar Large Field Camera imaging can be modeled as a single temperature blackbody with Tbb = 1.7 × 105 K and a bolometric luminosity of 3 × 10⁴⁵ 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) < 10⁴¹ 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 ≈10⁷M_⊙ 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.