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Stiele, Holger (2010): X-ray Source Population Study of the Local Group Galaxy M 31. Dissertation, LMU München: Faculty of Physics

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This dissertation presents the analysis of a large and deep XMM-Newton survey of the second large Local Group spiral galaxy M 31. The survey observations, taken between June 2006 and February 2008, together with re-analysed archival observations from June 2000 to July 2004 cover, for the first time, the whole D25 ellipse of M 31 with XMM-Newton down to a limiting luminosity of ~10^35 erg/s in the 0.2-4.5 keV band. The main goal of the thesis was a study of the different source populations of M 31 that can be observed in X-rays. Therefore a catalogue was created, which contains all 1948 sources detected in the 0.2-12.0 keV range. 961 of these sources were detected in X-rays for the first time. Source classification and identification was based on X-ray hardness ratios, spatial extent of the sources, and by cross correlating with catalogues in the X-ray, optical, infrared and radio wavelengths. An additional classification criterion was the long-term temporal variability of the sources in X-rays. This variability allows us to distinguish between X-ray binaries and active galactic nuclei. Furthermore, supernova remnant classifications of previous studies that did not use long-term variability as a classification criterion, could be validated. Including previous Chandra and ROSAT observations in the long-term variability study allowed me to detect additional transient or at least highly variable sources, which are good candidates for being X-ray binaries. Fourteen of the 40 supersoft source (SSS) candidates correlated with optical novae and therefore can be considered the supersoft emission of the optical novae. Among them is the first nova/SSS detected in a globular cluster of M 31. Correlations with previous ROSAT and Chandra studies revealed that only three SSSs are visible for at least one decade. This result underlines the strong long-term variability found for the class of SSSs. In addition the correlations demonstrated that strict selection criteria have to be applied to securely select SSSs. An investigation of the spatial distribution of the 25 supernova remnants (SNRs) and 37 SNR candidates showed that many of these sources are consistent with the location of the 10 kpc dust ring and other star forming regions in M 31. This connection between SNRs and star forming regions implies that most of the remnants are from type II supernovae. The brightest sources of M 31 belong to the class of X-ray binaries (XRBs). Ten low mass XRBs (LMXBs) and 26 LMXB candidates were identified based on their temporal variability. In addition 36 LMXBs and 17 LMXB candidates were identified due to correlations with globular clusters and globular cluster candidates. From the LMXBs located in globular clusters one is a black hole candidate and another a neutron star candidate. From optical and X-ray colour-colour diagrams, possible high mass XRB (HMXB) candidates were selected. Two of these candidates have an X-ray spectrum as is expected for an HMXB containing a neutron star primary. To investigate the logN-logS relations of sources in the field of M 31, a catalogue of sources detected in the 2.0-10.0 keV energy range was created. The slope of the logN-logS relation for the whole galaxy is consistent with the expectation for spiral galaxies (Colbert et al. 2004). Subtracting the background logN-logS relation, the region beyond the D25 ellipse still contains about 13 sources/deg^2 of M 31 with fluxes above the completeness limit of ~3.2x10^{-14} erg/cm^2/s (~2.3x10^36 erg/s at the distance of M 31). The radial dependence of the source distribution in M 31's disc can be well fitted with an exponential profile, for limiting fluxes of ~3.2x10^{-14} erg/cm^2/s and 10^{-13} erg/cm^2/s (~7.3x10^36 erg/s). About 60% of all sources with fluxes above 3.2x10^{-14} erg/cm^2/s are background sources. While the contribution of background sources lies at ~20% in the inner disc region, the fraction increases to >~80% in the outer areas of M 31. For the dust ring region, the slope of the logN-logS relation as well as the number of sources and their dependence on the star forming rate were consistent with the universal logN-logS relation predicted from theoretical considerations of HMXBs (Grimm et al. 2003). These findings propose that the dust ring region contains a population of HMXBs. A comparison of the number of X-ray binaries (XRBs) obtained from the logN-logS study to the ones listed in the source catalogue showed that many XRBs detected in the logN-logS study with fluxes between 10^{-13} erg/cm^2/s and 3.2x10^{-14} erg/cm^2/s, or (independent from the flux) those XRBs located in the inner disc of M 31, remain without XRB classification in the source catalogue. The results presented in this thesis gave us deeper insights in the properties of the population of X-ray sources in M 31. Nevertheless, about 65% of all sources detected in the field of M 31 can be classified as ``hard" sources only, i.e. it is not possible to decide whether these sources are X-ray binaries or Crab-like supernova remnants in M 31, or active galactic nuclei. Deeper observations in the X-ray and at other wavelengths are needed to classify these sources.