The Murmur of The Hidden Monster: Chandra's Decadal View of The Super-massive Black Hole in M31

Abstract

The Andromeda galaxy (M31) hosts a central super-massive black hole (SMBH), known as M31${}^{\ast }$, which is remarkable for its mass ($\sim$$10^8{\mathrm{M}}_{\odot }$) and extreme radiative quiescence. Over the past decade, the Chandra X-ray observatory has pointed to the center of M31 $\sim$100 times and accumulated a total exposure of $\sim$900 ks. Based on these observations, we present an X-ray study of a highly variable source that we associate with M31${}^{\ast }$ based on positional coincidence. We find that M31${}^{\ast }$ remained in a quiescent state from late 1999 to 2005, exhibiting an average 0.5-8 keV luminosity $\lesssim$$10^{36}\mathrm{ergs}{\mathrm{s}}^{-1}$, or only $\sim$$10^{-10}$ of its Eddington luminosity. We report the discovery of an outburst that occurred on January 6, 2006, during which M31${}^{\ast }$ radiated at $\sim$$4.3\times 10^{37}\mathrm{ergs}{\mathrm{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$$4.8\times 10^{36}\mathrm{ergs}{\mathrm{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.

13 pages, 3 figures, accepted by ApJL, revised version