On the metallicity dependence of high-mass X-ray binaries

It is commonly assumed that high-mass X-ray binary (HMXB) populations are little affected by metallicity. However, the massive stars making up their progenitor systems depend on metallicity in a number of ways, not least through their winds. We present simulations, well-matched to the observed sample of Galactic HMXBs, which demonstrate that both the number and the mean period of HMXB progenitors can vary with metallicity, with the number increasing by about a factor of 3 between solar and Small Magellanic Cloud (SMC) metallicity. However, the SMC population itself cannot be explained simply by metallicity effects; it requires both that the HMXBs observed therein primarily sample the older end of the HMXB population and that the star formation rate at the time of their formation was very large.