Low-mass X-ray binaries as a stellar mass indicator for the host galaxy

Using results of Chandra observations of old stellar systems in 11 nearby galaxies of various morphological types and the census of low-mass X-ray binaries (LMXBs) in the Milky Way, we study the population of LMXBs and their relation to the mass of the host galaxy. We show that the azimuthally averaged spatial distributions of the number of LMXBs and, in the majority of cases, of their collective luminosity closely follow that of the near-infrared light. Considering galaxies as a whole, we find that, in a broad stellar mass range, log(M-*) similar to9-11.5, the total number of LMXBs and their combined luminosity are proportional to the stellar mass of the host galaxy. Within the accuracy of the light-to-mass conversion, we cannot rule out the possibility of a weak dependence of the X/M-* ratio on morphological type. However, the effect of such a dependence, if any, does not exceed a factor of similar to1.5-2. The luminosity distributions of LMXBs observed in different galaxies are similar to each other and, with the possible exception of NGC 1553, are consistent with the average luminosity function derived from all data. The average X-ray luminosity function of LMXBs in nearby galaxies has a complex shape and is significantly different from that of high-mass X-ray binaries (HMXBs). It follows a power law with a differential slope of approximate to1 at low luminosities, gradually steepens at log(L-X) greater than or similar to 37.0-37.5 and has a rather abrupt cut-off at log(L-X) similar to39.0-39.5. This value of the cut-off luminosity is significantly, by an order of magnitude, lower than found for HMXBs.