2011ApJ...728L..10L

The Andromeda galaxy (M31) hosts a central supermassive black hole (SMBH), known as M31*, which is remarkable for its mass (∼10⁸ M_☉) and extreme radiative quiescence. Over the past decade, the Chandra X-Ray Observatory has pointed to the center of M31 ∼100 times and accumulated a total exposure of ∼900 ks. Based on these observations, we present an X-ray study of a highly variable source that we associate with M31* based on positional coincidence. We find that M31* remained in a quiescent state from late 1999 to 2005, exhibiting an average 0.5-8 keV luminosity ≲ 10³⁶ erg/s, or only ∼10^–10 of its Eddington luminosity. We report the discovery of an outburst that occurred on 2006 January 6 during which M31* radiated at ∼4.3x10³⁷ erg/s. After the outburst, M31* entered a more active state that apparently lasts to the present, which is characterized by frequent flux variability around an average luminosity of ∼4.8x10³⁶ erg/s. These flux variations are similar to the X-ray flares found in the SMBH of our Galaxy (Sgr A*), making M31* the second SMBH known to exhibit recurrent flares. Future coordinated X-ray/radio observations will provide useful constraints on the physical origin of the flaring emission and help rule out a possible stellar origin of the X-ray source.