Bright end of the luminosity function of high-mass X-ray binaries: contributions of hard, soft and supersoft sources

TL;DR

This study constructs the intrinsic X-ray luminosity function of high-mass X-ray binaries in nearby galaxies, revealing their spectral diversity and correcting for absorption effects, with implications for understanding early Universe X-ray preheating.

Contribution

It provides the first detailed intrinsic XLF of luminous HMXBs, accounting for spectral diversity and absorption, and compares the hard, soft, and supersoft source populations.

Findings

Intrinsic HMXB number per SFR is ~2.3 times larger than previously observed.

The intrinsic XLF follows a power law with specific parameters from 10^38 to 10^40.5 erg/s.

Hard, soft, and supersoft sources are present in roughly equal numbers in the soft X-ray band.

Abstract

Using a spectral analysis of bright Chandra X-ray sources located in 27 nearby galaxies and maps of star-formation rate (SFR) and ISM surface densities for these galaxies, we constructed the intrinsic X-ray luminosity function (XLF) of luminous high-mass X-ray binaries (HMXBs), taking into account absorption effects and the diversity of HMXB spectra. The XLF per unit SFR can be described by a power law dN/d log L=2.0(L/10^39 erg/s)^(-0.6) (M_Sun/yr)^(-1) from L=10^38 to 10^40.5 erg/s, where L is the unabsorbed luminosity at 0.25-8 keV. The intrinsic number of luminous HMXBs per unit SFR is a factor of ~2.3 larger than the observed number reported before. The intrinsic XLF is composed of hard, soft and supersoft sources (defined here as those with the 0.25-2 keV to 0.25-8 keV flux ratio of <0.6, 0.6-0.95 and >0.95, respectively) in ~2:1:1 proportion. We also constructed the intrinsic HMXB XLF in the soft X-ray band (0.25-2 keV). Here, the numbers of hard, soft and supersoft sources prove to be nearly equal. The cumulative present-day 0.25-2 keV emissivity of HMXBs with luminosities between 10^38 and 10^40.5 erg/s is ~5 10^39 erg/s (M_Sun/yr)^(-1), which may be relevant for studying the X-ray preheating of the early Universe.