The X-ray source population of the Andromeda galaxy M 31

First studies of the X-ray source population of M 31 were performed with the Einstein Observatory and ROSAT. High resolution Chandra Observatory images not only spatially resolved the center area but also supernova remnants (SNRs) in the galaxy. Source catalogues of restricted areas were presented with high astrometric accuracy. Also luminosity function studies and studies of individual sources based on Chandra and XMM-Newton observations led to a better knowledge of the X-ray source population. An XMM-Newton source catalog based on archival observations revealed more than 850 sources down to a 0.2-4.5 keV luminosity of 10(35) erg s(-1). EPIC hardness ratios as well as informations from earlier X-ray, optical, and radio catalogues were used to distinguish between different source classes (SNRs, supersoft sources (SSSs), X-ray binaries (XRBs), globular cluster sources within M 31, and foreground stars and objects in the background). However, many sources could only be classified as hard. These sources may either be XRBs or Crab-like SNRs in M 31 or background sources. Two of the globular cluster sources could be identified as low mass XRBs with a neutron star as compact object as they showed type I X-ray bursts. Many of the SSSs were identified as optical novae. Inspired by these results an XMM-Newton survey of the entire D-25 disk of M 31 and a dedicated program to monitor X-ray counterparts of optical novae in M 31 was started. We discuss implications for further nearby galaxy studies. (C) 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.