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

Abstract

Using results of Chandra observations of old stellar systems in eleven nearby galaxies of various morphological types and the census of LMXBs in the Milky Way, we study the population of low mass X-ray binaries and their relation to the mass of the host galaxy. We show that the azimuthally averaged spatial distribution of the number of LMXBs and, in the majority of cases, of their collective luminosity closely follows that of the near-infrared light. Considering galaxies as a whole, we find that in a broad mass range, log(M)~9-11.5, 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 ~1.5-2. The luminosity distributions of LMXBs observed in different galaxies are similar to each other and, with the possible exception of NGC1553, are consistent with the average luminosity function derived from all data. The average XLF of LMXBs in nearby galaxies has a complex shape and is significantly different from that of HMXBs. It follows a power law with a differential slope of ~1 at low luminosities, gradually steepens at log(Lx)>37.0-37.5 and has a rather abrupt cut-off at log(Lx)~39.0-39.5. This value of the cut-off luminosity is significantly, by an order of magnitude, lower than found for high mass X-ray binaries.