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

Abstract

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 ∼10 37 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′, 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′ from the centre of M 31 are presumably created via dynamical interactions. The dynamical origin of the r < 1′ 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(Lx) ≲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. © ESO 2007.