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

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 −1 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.

X-ray detected AGN in SDSS dwarf galaxies

“…Thus the Mineo et al (2012a) XLF could be underestimating the probability of finding a HMXB in this population of galaxies. Lehmer et al (2019) observes this enhancement in 4 dwarf galaxies of similar masses and metallicities to those in our sample. We follow the same procedure outlined earlier in the paragraph but use the normalisations outlined in Lehmer et al (2019) and observe some increase in the probability of finding a HMXB at any given X-ray luminosity.…”

“…Lehmer et al (2019) observes this enhancement in 4 dwarf galaxies of similar masses and metallicities to those in our sample. We follow the same procedure outlined earlier in the paragraph but use the normalisations outlined in Lehmer et al (2019) and observe some increase in the probability of finding a HMXB at any given X-ray luminosity. We have not shown this relationship on figure 9 because the small sample size means the relationship will have large and undefined uncertainties and believe plotting the relationship would appear overly definitive.…”

The next-generation X-ray galaxy survey with eROSITA

“…Recent work has shown that the galaxy-to-galaxy scatter from these relations may be due to physical differences between the galaxies, i.e. stellar ages (Fragos et al 2008;Lehmer et al 2017Lehmer et al , 2019 or metallicity (Fragos et al 2013;Basu-Zych et al 2013a;Douna et al 2015;Brorby et al 2016;Lehmer et al 2016), yet this level of detail is beyond the scope of this paper and does not affect the overall predictions. However, our estimates indicate that the 4-year eROSITA survey will detect sufficient numbers of galaxies to allow detailed investigation of X-ray emission as a function of e.g.…”

“…In practice, galaxy-to-galaxy variations from these scaling relations can be quite large, and have been shown to vary systematically with second-order galaxy properties such as metallicity and stellar age (e.g. Basu-Zych et al 2013a,b;Douna et al 2015;Brorby et al 2016;Lehmer et al 2016Lehmer et al , 2017Lehmer et al , 2019. In this section we explore the effect of scatter from the above scaling relations on our predictions of whether a galaxy can be detected.…”

A Second Look at 12 Candidate Dual AGNs Using BAYMAX

“…As an additional step in the analysis of the true nature of the secondary point source, we compare the hard X-ray luminosity to the total, expected X-ray luminosity due to XRBs. Following a similar analysis to Foord et al (2017a), we adopt an updated analytic prescription by Lehmer et al (2019), to estimate the total, 2-7 keV luminosity expected from XRBs (L gal XRB ). In particular, for a given stellar mass (M * , which scales with the LMXB population; Gilfanov 2004) and star formation rate (SFR, which scales with the HMXB population; Mineo et al 2012) the total, 2-7 keV luminosity from XRBs can be estimated (Lehmer et al 2019).…”