The Dependence of the Soft X-Ray Properties of Low-Mass X-Ray Binaries on the Metallicity of Their Environment

Abstract

We determine the X-ray spectral properties of a sample of low-mass X-ray binaries (LMXBs) that reside in globular clusters of M31 as well as five LMXBs in Galactic globular clusters and in the Large Magellanic Cloud using the ROSAT Position Sensitive Proportional Counter. We find a trend in the X-ray properties of the LMXBs as a function of globular cluster metallicity. The spectra of LMXBs become progressively softer as the metallicity of their environment increases. The one M31 globular cluster LMXB in our sample that has a metallicity greater than solar has spectral properties similar to those of LMXBs in the bulge of M31 but markedly different from those that reside in low-metallicity globular clusters, both in M31 and the Galaxy. The spectral properties of this high-metallicity LMXB are also similar to those of X-ray-faint, early-type galaxies. This lends support to the claim that a majority of the X-ray emission from these X-ray-faint, early-type galaxies results from LMXBs and not hot gas, as is the case in their X-ray-bright counterparts.