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 , \approx 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 \times 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 \times 10 ^ { 5 } K and a bolometric luminosity of 3 \times 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 \approx 10 ^ { 7 } M _ { \odot } 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 .