An X-ray spectral survey of the disc of M 31 with XMM-NewtonL. Shaw Greening1, R. Barnard1, U. Kolb1, C. Tonkin1, and J. P. Osborne2
1 The Department of Physics and Astronomy, The Open University, Walton Hall, Milton Keynes, MK7 6AA, UK
2 The Department of Physics and Astronomy, The University of Leicester, Leicester, LE1 7RH, UK
Received 27 March 2008 / Accepted 15 July 2008
Aims. We present the results of a complete spectral survey of the X-ray point sources detected in five XMM-Newton observations along the major axis of M 31 but avoiding the central bulge, aimed at establishing the population characteristics of X-ray sources in this galaxy.
Methods. We obtained background subtracted spectra and lightcurves for each of the 335 X-ray point sources detected across the five observations from 2002. We also correlate our source list with those of earlier X-ray surveys and radio, optical and infra-red catalogues. Sources with more than 50 source counts are individually spectrally fit in order to create the most accurate luminosity functions of M 31 to date.
Results. Based on the spectral fitting of these sources with a power law model, we observe a broad range of best fit photon index. From this distribution of best fit index, we identify 16 strong high mass X-ray binary system candidates in M 31. We show the first cumulative luminosity functions created using the best fit spectral model to each source with more than 50 source counts in the disc of M 31. The cumulative luminosity functions show a distinct flattening in the X-ray luminosity interval 37.0 log erg s-1 37.5. Such a feature may also be present in the X-ray populations of several other galaxies, but at a much lower statistical significance. We investigate the number of AGN present in our source list and find that above ~1.41036 erg s-1 the observed population is statistically dominated by the point source population of M 31.
Key words: galaxies: individual: M 31 -- X-rays: general -- X-rays: binaries
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