NuSTAR view of the central region of M31

Abstract

Our neighbouring large spiral galaxy, the Andromeda galaxy (M31 or NGC 224), is an ideal target to study the X-ray source population of a nearby galaxy. NuSTAR observed the central region of M31 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 M31, 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.