Galaxy Science with STAR-X: Exploring the X-ray binary populations near and far

The high sensitivity of STAR-X combined with its high observing efficiency open a new window for studies of the X-ray binary populations (XRB) from our nearest galaxies to cosmologically interesting distances. High-cadence observations of a representative sample of nearby galaxies will constrain for the first time the variability timescales and duty cycles of X-ray binaries in different galactic environments. We will describe a two-year long monitoring campaign of a few nearby galaxies spanning the full range of specific star-formation rates, from early-type galaxies (dominated by Low-Mass X-ray binaries) to intensely star-forming galaxies (dominated by High-Mass X-ray binaries). These observations will be scheduled following a logarithmic cadence, allowing us to measure their variability and outburst timescales, thereby detecting and characterizing the bright end of the XRB luminosity function (e.g. outbursting XRBs, and ultraluminous X-ray sources, ULXs). Combining these observations we will also study the diffuse emission of these galaxies and obtain spectral maps of their hot Interstellar medium (ISM) and the relative contribution of the XRB and hot gas components in different regions of the galaxies. Comparison of these maps with probes of the other ISM phases (e.g. optical spectral line maps, IR continuum, 21 cm) will give us an excellent picture of the multiphase ISM and its relation to the galactic star-forming activity. The STAR-X nearby galaxy survey will serve as a benchmark for the study of the cosmological evolution of the galactic X-ray emission which will be achieved with the medium and deep X-ray surveys performed with STAR-X. These surveys will detect galaxies at sensitivity levels ~1dex deeper than the eROSITA and the COSMOS surveys respectively, allowing us to measure the cosmological evolution of the galactic X-ray emission with higher precision than is currently possible, and explore the role of metallicity in the enhanced X-ray output of higher-redshift galaxies.