Decomposition of galactic X-ray emission with Phox: Contributions from hot gas and XRBs

TL;DR

This paper introduces a numerical framework using hydrodynamical simulations and the Phox code to accurately model and analyze the contributions of hot gas and X-ray binaries to galactic X-ray emission, aligning well with observations.

Contribution

It extends the Phox code to include stellar components and demonstrates a comprehensive, spectrally accurate method for studying galactic X-ray emission from cosmological simulations.

Findings

Reproduces X-ray luminosity functions up to the one-photon limit.

Finds a significant overlap between simulated and observed $L_{X}$-SFR-$M_{\star}$ relations.

Shows spatial distributions of X-ray binaries match star-formation and stellar mass regions.

Abstract

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 ($M_{\star}$) as well as star-formation rate (SFR). Using simulated galaxy halos extracted from the $(48\,h^{-1} \mathrm{cMpc})^3$ 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. Average X-ray luminosity functions are perfectly reproduced up to the one-photon luminosity limit. Comparing our resulting $L_{X}-\mathrm{SFR}-M_{\star}$ 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 anti-correlation 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 2-10 keV band in the absence of an actively accreting central super-massive black hole with 50% contribution in the 0.5-2 keV band rivaling the hot gas component. Our modelling remains consistent with observations despite uncertainties connected to our approach. The predictive power and easily extendable framework hold great value for future investigations of galactic X-ray spectra.