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