Decomposition of galactic X-ray emission with PHOX. Contributions from hot gas and X-ray binaries

Context. X-ray observations of galaxies with high spatial resolution instruments such as Chandra have revealed that major contributions to their diffuse emission originate from X-ray-bright point sources in the galactic stellar field. It has been established that these point sources, called X-ray binaries, are accreting compact objects with stellar donors in a binary configuration. They are classified according to the predominant accretion process: wind-fed in the case of high-mass donors and Roche-lobe mass transfer in the case of low-mass donors. Observationally, it is challenging to reliably disentangle these two populations from each other because of their similar spectra.
Aims: We provide a numerical framework with which spatially and spectrally accurate representations of X-ray binary populations can be studied from hydrodynamical cosmological simulations. We construct average spectra, accounting for a hot gas component, and verify the emergence of observed scaling relations between galaxy-wide X-ray luminosity (L_X) and stellar mass (M_*) and between L_X and the star-formation rate (SFR).
Methods: Using simulated galaxy halos extracted from the (48 h⁻¹ cMpc)³ volume of the Magneticum Pathfinder cosmological simulations at z = 0.07, we generate mock spectra with the X-ray photon-simulator PHOX. We extend the PHOX code to account for the stellar component in the simulation and study the resulting contribution in composite galactic spectra.
Results: Well-known X-ray binary scaling relations with galactic SFR and M_* emerge self-consistently, verifying our numerical approach. Average X-ray luminosity functions are perfectly reproduced up to the one-photon luminosity limit. Comparing our resulting L_X − SFR − M_* relation for X-ray binaries with recent observations of field galaxies in the Virgo galaxy cluster, we find significant overlap. Invoking a metallicity-dependent model for high-mass X-ray binaries yields an anticorrelation between mass-weighted stellar metallicity and SFR-normalized luminosity. The spatial distribution of high-mass X-ray binaries coincides with star-formation regions of simulated galaxies, while low-mass X-ray binaries follow the stellar mass surface density. X-ray binary emission is the dominant contribution in the hard X-ray band (2-10 keV) in the absence of an actively accreting central super-massive black hole, and it provides a ∼50% contribution in the soft X-ray band (0.5-2 keV), rivaling the hot gas component.
Conclusions: We conclude that our modeling remains consistent with observations despite the uncertainties connected to our approach. The predictive power and easily extendable framework hold great value for future investigations of galactic X-ray spectra.