# X-ray Binary Luminosity Function Scaling Relations for Local Galaxies Based on Subgalactic Modeling

## TL;DR

This study uses Chandra data to analyze X-ray binary luminosity functions across 38 nearby galaxies, revealing complex dependencies on galaxy properties like star formation rate, stellar mass, metallicity, and globular cluster content.

## Contribution

It introduces a global XLF model that accounts for contributions from HMXBs, LMXBs, and background sources, and explores their dependence on galaxy-scale parameters.

## Key findings

- HMXB XLF shape is more complex than previously thought.
- LMXB XLF likely varies with specific SFR, indicating age dependence.
- Galaxies with low metallicity or rich globular clusters show deviations from the global model.

## Abstract

We present new Chandra constraints on the X-ray luminosity functions (XLFs) of X-ray binary (XRB) populations, and their scaling relations, for a sample of 38 nearby galaxies (D = 3.4-29 Mpc). Our galaxy sample is drawn primarily from the Spitzer infrared nearby galaxy survey (SINGS), and contains a wealth of Chandra (5.8 Ms total) and multiwavelength data, allowing for star-formation rates (SFRs) and stellar masses (M*) to be measured on subgalactic scales. We divided the 2478 X-ray detected sources into 21 subsamples in bins of specific-SFR (sSFR = SFR/M*) and constructed XLFs. To model the XLF dependence on sSFR, we fit a global XLF model, containing contributions from high-mass XRBs (HMXBs), low-mass XRBs (LMXBs), and background sources from the cosmic X-ray background (CXB) that respectively scale with SFR, M*, and sky area. We find an HMXB XLF that is more complex in shape than previously reported and an LMXB XLF that likely varies with sSFR, potentially due to an age dependence. When applying our global model to XLF data for each individual galaxy, we discover a few galaxy XLFs that significantly deviate from our model beyond statistical scatter. Most notably, relatively low-metallicity galaxies have an excess of HMXBs above ~10^38 erg/s and elliptical galaxies that have relatively rich populations of globular clusters (GCs) show excesses of LMXBs compared to the global model. Additional modeling of how the XRB XLF depends on stellar age, metallicity, and GC specific frequency is required to sufficiently characterize the XLFs of galaxies.