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

TL;DR

This study analyzes a decade of Chandra X-ray observations of M31's supermassive black hole, revealing a quiescent state, a significant outburst in 2006, and ongoing recurrent flares similar to those in our Galaxy's SMBH.

Contribution

First detailed long-term X-ray monitoring of M31's SMBH, identifying its quiescent, outburst, and active flaring states, and comparing its behavior to Sgr A*.

Findings

M31* was quiescent from 1999 to 2005 with luminosity ≤10^36 erg/s.

A major outburst occurred on January 6, 2006, reaching 4.3×10^37 erg/s.

Post-outburst, M31* shows recurrent flux variability similar to Galactic SMBH flares.

Abstract

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