An Explanation for Metallicity Effects on X-Ray Binary Properties

We show that irradiation-induced stellar winds can explain two important metallicity effects in X-ray binaries: the higher numbers and softer spectra of X-ray binaries in metal-rich globular clusters (GCs) compared to those in metal-poor ones. As has been previously noted by Iben, Tutukov, and Fedorova, the winds should be stronger at lower metallicity because of less efficient line cooling. This speeds up the evolution of low-mass X-ray binaries (LMXBs) in metal-poor clusters and hence reduces their number. These winds can also provide extra material near the accreting object, which can create an intrinsic absorber that hardens the X-ray spectra of the metal-poor cluster systems relative to the metal-rich ones, as suggested by observations. We outline some additional observational predictions of this model.