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

Shaw Greening, L.; Barnard, R.; Kolb, U.; Tonkin, C. and Osborne, J. P. (2009). An X-ray spectral survey of the disc of M 31 with XMM-Newton. Astronomy & Astrophysics, 495(3) pp. 733–741.

DOI: https://doi.org/10.1051/0004-6361/200809864

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 M31 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 M31 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 M31. 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 M31. 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 ~ 1.4 × 10³⁶ erg s⁻¹ the observed population is statistically dominated by the point source population of M31.

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