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 × 10 43 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 T bb = 1.7 × 10 5 K and a bolometric luminosity of 3 × 10 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 ☉ 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.