Fifteen Years of X-Rays from Andromeda’s Supermassive Black Hole

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

Essentially all massive galaxies are expected to harbor a supermassive black hole. The properties of supermassive black holes are varied, spanning a wide range of masses and activity levels. The two best-studied black holes fall at opposite ends of the mass spectrum: the Milky Way’s, at 4 million solar masses, and Messier 87’s, at 6 billion solar masses.

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.

This task is easier said than done, as the supermassive black hole isn’t the only source of X-rays in Andromeda’s center. Andromeda’s center contains a crowded cluster of four X-ray sources named P2, N1, S1, and SSS. P2 is associated with the supermassive black hole, and the nature of the remaining three sources is unknown. Because these sources are so close to one another, their emission overlaps, and each source varies on its own individual timescale. DiKerby’s team simultaneously modeled the flux from each source, reconstructing their individual light curves and extracting the X-ray behavior of the black hole.

Staying Active

The light curve for Andromeda’s supermassive black hole shows a roughly constant count rate punctuated by a flare in 2013. The count rate from 2009 to 2016 is a continuation of the elevated flux state that began after a flare in 2006. The gap in observations between 2016 and 2021 makes it difficult to say for sure, but observations from 2022 to the present suggest that the black hole may still be in an elevated flux state today. Given the gaps in the data, it’s impossible to know if the elevated flux observed today is due to the flare seen in 2006, or if it’s due to subsequent flares.

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