Decomposition of galactic X-ray emission with PHOX

Contributions from hot gas and X-ray binaries

¹ Universitäts-Sternwarte, Fakultät für Physik, Ludwig-Maximilians-Universität München, Scheinerstr.1, 81679 München, Germany
e-mail: vladutescu@usm.lmu.de
² INAF, Osservatorio Astronomico di Trieste, via Tiepolo 11, 34131 Trieste, Italy
³ Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Straße 1, 85748 Garching bei München, Germany

Received: 9 August 2022
Accepted: 18 October 2022

Abstract

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.