The Murmur of The Hidden Monster: Chandras Decadal View of The Super-massive Black Hole in M31

The Andromeda galaxy (M31) hosts a central super-massive black hole (SMBH), known as M31$^ast$, which is remarkable for its mass ($sim$$10^8{rm~M_odot}$) and extreme radiative quiescence. Over the past decade, the Chandra X-ray observatory has pointed to the center of M31 $sim$100 times and accumulated a total exposure of $sim$900 ks. Based on these observations, we present an X-ray study of a highly variable source that we associate with M31$^ast$ based on positional coincidence. We find that M31$^ast$ remained in a quiescent state from late 1999 to 2005, exhibiting an average 0.5-8 keV luminosity $lesssim$$10^{36}{rm~ergs~s^{-1}}$, or only $sim$$10^{-10}$ of its Eddington luminosity. We report the discovery of an outburst that occurred on January 6, 2006, during which M31$^ast$ radiated at $sim$$4.3times10^{37}{rm~ergs~s^{-1}}$. After the outburst, M31$^ast$ entered a more active state that apparently lasts to the present, which is characterized by frequent flux variability around an average luminosity of $sim$$4.8times10^{36}{rm~ergs~s^{-1}}$. These flux variations are similar to the X-ray flares found in the SMBH of our Galaxy (Sgr A$^ast$), making M31$^ast$ 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.