2025MNRAS.542.2087B

X-ray binaries play a significant role in the thermal and ionization history of galaxies. Their X-ray luminosity can shed light on galactic star formation rates and histories. Compact objects are also crucial in the evolution of gravitational wave progenitors. Here, we present the results from our work to extend the binary population and spectral synthesis (BPASS) code suite to incorporate X-ray emission on to compact remnants in binary systems. We self-consistently model the accretion disc for each interacting binary system in a grid of stellar evolution models and then combine these to obtain the total X-ray spectra for stellar populations over a range of ages and metallicities. Crucially, these are estimated using the same stellar models as those used for modelling the stellar spectral energy distribution. We utilize first principle equations to calculate the X-ray binary (XRB) evolution, luminosity and spectral energy densities of individual accreting compact objects. Population synthesis using observationally motivated values for $R_{\textrm {inner}}$ (the accretion disc inner truncation radius) reproduces the observed X-ray number evolution in the Small Magellanic Cloud and the inferred X-ray flux evolution for M51, validating our models. Using these models, we explore the implications of a self-consistent stellar and XRB emission population synthesis for ionizing photon production, the XRB dependence on metallicity and, for XRBs as a potential source of nebular He II emission seen in the spectra of high-redshift galaxies. We conclude that XRBs contribute towards powering nebular He II emission without causing significant overestimates of hydrogen ionization.