A study of the population of LMXBs in the bulge of M 31

Aims: We explore the population of X-ray point sources in the bulge of M 31 to contrast properties of various subpopulations, such as persistent and transient sources and primordial LMXBs and dynamically formed ones.
Methods: Based on the data from 26 archival Chandra observations we study the source content and properties of various subpopulations of X-ray sources to a maximum distance of 12´ from the centre of M 31.
Results: To a limiting luminosity of ∼ 10³⁵ erg s^-1 we find 263 X-ray point sources, with ∼ 1/3 of these being background galaxies. A study of the spatial distribution and the luminosity function of the X-ray sources shows that the distribution of primordial LMXBs is consistent with the distribution of the K-band light and that their luminosity function flattens below ∼ 10³⁷ erg s^-1 to the dN/dL∝ L^-1 law in agreement with the behaviour found earlier for LMXBs in the Milky Way and in Cen A. Within a radius of 12 arcmin, the luminosity function is independent of distance to the centre of M 31, in contrast to earlier Chandra studies. The LMXBs located in globular clusters and within ∼ 1 arcmin from the centre of M 31 are presumably created via dynamical interactions. The dynamical origin of the r< 1 arcmin sources is strongly suggested by their radial distribution which follows the ρ^2_{*} profile rather than the K-band light distribution. Their luminosity function shows a prominent fall-off below log(L_X)⪉ 36.5. Although the statistics is insufficient to claim a genuine low-luminosity cut-off in the luminosity function, the best fit powerlaw with a slope of -0.6±0.2 is significantly flatter than the dN/dL∝ L^-1 law. We also searched for transients and found 28 sources that varied by a factor larger than 20. Their spatial distribution follows the distribution of the persistent LMXBs within the accuracy allowed by the limited number of transients.

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