NuSTAR view of the central region of M 31

TL;DR

This study uses NuSTAR observations to analyze the high-energy X-ray sources in the central region of the Andromeda galaxy, identifying potential X-ray binaries and examining their spectral and variability properties.

Contribution

First NuSTAR-based analysis of M 31's central X-ray sources at energies above 10 keV, identifying candidate X-ray binaries and characterizing their spectral and temporal behavior.

Findings

Identified four potential X-ray binary candidates.

Detected low but significant long-term variability in seven sources.

Analyzed spectral properties and hardness ratios of X-ray sources.

Abstract

Our neighbouring large spiral galaxy, the Andromeda galaxy (M 31 or NGC 224), is an ideal target to study the X-ray source population of a nearby galaxy. NuSTAR observed the central region of M 31 in 2015 and allows studying the population of X-ray point sources at energies higher than 10 keV. Based on the source catalogue of the large XMM-Newton survey of M 31, we identified counterparts to the XMM-Newton sources in the NuSTAR data. The NuSTAR data only contain sources of a brightness comparable (or even brighter) than the selected sources that have been detected in XMM-Newton data. We investigate hardness ratios, spectra and long-term light curves of individual sources obtained from NuSTAR data. Based on our spectral studies we suggest four sources as possible X-ray binary candidates. The long-term light curves of seven sources that have been observed more than once show low (but significant) variability.