An X-ray spectral survey of the disc of M 31 with XMM-Newton^*

¹ The Department of Physics and Astronomy, The Open University, Walton Hall, Milton Keynes, MK7 6AA, UK e-mail: L.shaw-greening@open.ac.uk
² The Department of Physics and Astronomy, The University of Leicester, Leicester, LE1 7RH, UK

Received: 27 March 2008
Accepted: 15 July 2008

Abstract

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 L_X interval 37.0 ≲ log L_X 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 L_X~1.410³⁶ erg s^-1 the observed population is statistically dominated by the point source population of M 31.