Fifteen Years of M31* X-ray Variability and Flares

Abstract

We append an additional fifteen years (2009-2024) to the Chandra X-ray light curve of M31*, the supermassive black hole at the center of M31, the Andromeda galaxy. Extending and expanding on the work in Li et al. 2011, we show that M31* has remained in an elevated X-ray state from 2006 through at least 2016 (when regular Chandra monitoring ceased) and likely through 2024, with the most recent observations still showing an elevated X-ray flux. We identify one moderate flare in 2013 where the other nuclear X-ray sources are in low-flux states, making that flare a valuable target for followup with multiwavelength and multimessenger archival data. We extract a mostly uncontaminated spectrum for M31* from this observation, showing that its X-ray properties are similar to those observed at Sgr A* in its quiescent state by Baganoff et al. 2003. Furthermore, we find no substantial change in the source's hardness ratio in the 2006 and 2013 flares compared to the post-2006 elevated state, suggesting the these flares are increases in the regular X-ray emission mechanisms instead of entirely new emission modes. Our extended light curve for M31* provides valuable context for multimessenger or multiwavelength observations of nearby supermassive black holes.