Luminosity functions of LMXBs in different stellar environments

Aims: Based on the archival data from the Chandra observations of nearby galaxies, we study different sub populations of low-mass X-ray binaries (LMXBs) - dynamically formed systems in globular clusters (GCs) and in the nucleus of M 31 and (presumably primordial) X-ray binaries in the fields of galaxies. Our aim is to produce accurate luminosity distributions of X-ray binaries in different environments, suitable for quantitative comparison with each other and with the output of population synthesis calculations.
Methods: Our sample includes seven nearby galaxies (M 31, Maffei 1, Centaurus A, M 81, NGC 3379, NGC 4697, and NGC 4278) and the Milky Way, which together provide relatively uniform coverage down to the luminosity limit of 10³⁵ erg s^-1. In total we have detected 185 LMXBs associated with GCs, 35 X-ray sources in the nucleus of M 31, and 998 field sources of which ~365 are expected to be background AGN. We combine these data, taking special care to accurately account for X-ray and optical incompleteness corrections and the removal of the contamination from the cosmic X-ray background sources, to produce luminosity distributions of X-ray binaries in different environments to far greater accuracy than has been obtained previously.
Results: We found that luminosity distributions of GC and field LMXBs differ throughout the entire luminosity range, the fraction of faint (log (L_X) < 37) sources among the former being ~4 times less than in the field population. The X-ray luminosity function (XLF) of sources in the nucleus of M 31 is similar to that of GC sources at the faint end but differs at the bright end, with the M 31 nucleus hosting significantly fewer bright sources. We discuss the possible origin and potential implications of these results.

Tables 2 and 6 are available in electronic form at http://www.aanda.org