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

摘要

The Andromeda galaxy (M31) hosts a central super-massive black hole (SMBH),known as M31$^\ast$, which is remarkable for its mass($\sim$$10^8{\rm~M_\odot}$) and extreme radiative quiescence. Over the pastdecade, the Chandra X-ray observatory has pointed to the center of M31$\sim$100 times and accumulated a total exposure of $\sim$900 ks. Based onthese observations, we present an X-ray study of a highly variable source thatwe associate with M31$^\ast$ based on positional coincidence. We find thatM31$^\ast$ remained in a quiescent state from late 1999 to 2005, exhibiting anaverage 0.5-8 keV luminosity $\lesssim$$10^{36}{\rm~ergs~s^{-1}}$, or only$\sim$$10^{-10}$ of its Eddington luminosity. We report the discovery of anoutburst that occurred on January 6, 2006, during which M31$^\ast$ radiated at$\sim$$4.3\times10^{37}{\rm~ergs~s^{-1}}$. After the outburst, M31$^\ast$entered a more active state that apparently lasts to the present, which ischaracterized by frequent flux variability around an average luminosity of$\sim$$4.8\times10^{36}{\rm~ergs~s^{-1}}$. These flux variations are similar tothe X-ray flares found in the SMBH of our Galaxy (Sgr A$^\ast$), makingM31$^\ast$ the second SMBH known to exhibit recurrent flares. Futurecoordinated X-ray/radio observations will provide useful constraints on thephysical origin of the flaring emission and help rule out a possible stellarorigin of the X-ray source.