The Chandra X-ray Observatory has been keeping tabs on the Andromeda Galaxy for years. What do these observations tell us about the supermassive black hole at the center of our nearest major galactic neighbor?
Examining a Moderate Black Hole

Illustration of the relative sizes of the supermassive black holes at the center of the Milky Way (Sgr A*) and Messier 87 (M87*). Click to enlarge. [EHT collaboration (acknowledgment: Lia Medeiros, xkcd); CC BY 4.0]
The Milky Way’s nearest massive galactic neighbor, the Andromeda Galaxy, provides an opportunity to study a supermassive black hole that falls between these two mass extremes. Andromeda’s central supermassive black hole has a moderate mass of roughly 100 million solar masses and is known to exhibit flaring behavior. The Chandra X-ray Observatory has provided a wealth of X-ray observations of Andromeda’s center since 2000. What do these data tell us about what Andromeda’s black hole has been up to recently?
Journey to Center of Andromeda
In a recent publication, Stephen DiKerby (Michigan State University) and collaborators examined the behavior of Andromeda’s central supermassive black hole over the past 15 years. The team amassed a large sample of Chandra observations of Andromeda’s center from 2009 to the present, including all of the observations in the public archive. These data were stitched into an extensive light curve to investigate the supermassive black hole’s behavior.

Stacked image from Chandra’s High Resolution Camera showing the locations of the four X-ray sources near Andromeda’s center. [DiKerby et al. 2025]
Staying Active

Count rate for the X-ray source P2 over the past 15 years. An X-ray flare is highlighted in red. Click to enlarge. [DiKerby et al. 2025]
Another aspect of the black hole’s X-ray emission is its hardness ratio: a measure of whether more high-energy or low-energy X-rays were emitted. The team found that the hardness ratio was about the same during the 2013 flare as it was during non-flaring times. This suggests that the emission mechanism is the same for both flaring and non-flaring states.
Interestingly, the emission from Andromeda’s black hole has a similar hardness ratio to the Milky Way’s black hole when both black holes are quiescent, but the flares from Andromeda’s black hole are significantly softer (i.e., they have a greater proportion of low-energy X-rays) than those from the Milky Way’s black hole. This suggests that more investigation of the flaring mechanisms of the two black holes is needed, as is continued monitoring of the black hole at the heart of our neighboring galaxy.
Citation
“Fifteen Years of M31* X-Ray Variability and Flares,” Stephen DiKerby et al 2025 ApJ 981 50. doi:10.3847/1538-4357/adb1d5