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

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.