Constructing X-ray Spectral Models of Galaxies: Varying Contributions from X-ray Binary Populations with Host Galaxy Properties

Abstract

Recent work has shown that the emission from X-ray binary (XRB) populations in galaxies varies with stellar mass ($M_{\star }$), star formation rate (SFR), and metallicity ($Z$). Such scaling relations are widely used to predict the XRB contributions to galaxy-integrated X-ray luminosities including studies focused on dwarf active galactic nuclei (AGN) and the X-ray radiation field during the epoch of heating in the early ($z\ge 8$) universe. However, as galaxies approach low SFR and low $Z$, the relatively shallow slope of the XRB luminosity function (XLF) can yield very large stochastic variations in the total X-ray luminosity expected from the XRB population, for fixed values of $M_{\star }$, SFR, and $Z$. We have created a procedure to statistically sample any XLF and model total X-ray spectra for XRB populations and their stochastic uncertainties. We demonstrate the accuracy of this procedure using data for galaxies ranging from high to low $M_{\star }$, SFR, and $Z$ and generating X-ray spectral models consistent with Chandra observations. For galaxies that lie on the galactic main-sequence, we can relate SFR and $Z$ to $M_{\star }$ using established $M_{\star }$-SFR and $M_{\star }$-$Z$ relations. Applying these relations, we construct main-sequence (MS) XRB spectral models, which provide typical XRB spectral shapes, normalizations, and uncertainties as a function of $M_{\star }$. The spectral model library associated with this work is available at https://doi.org/10.5281/zenodo.20126734.

Accepted for publication in the Astrophysical Journal (ApJ). 19 pages, 14 figures, 4 tables