The Murmur of The Hidden Monster: Chandra's Decadal View of The Super-massive Black Hole in M31
Zhiyuan Li, Michael R. Garcia, William R. Forman, Christine Jones,, Ralph P. Kraft, Dharam V. Lal (CfA), Stephen S. Murray (CfA/JHU), Q. Daniel, Wang (UMass)

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, which is remarkable for its mass () 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 , or only of its Eddington luminosity. We report the discovery of an outburst that occurred on January 6, 2006, during which M31 radiated at . After the outburst, M31 entered a…
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