The intrinsic collective X-ray spectrum of luminous high-mass X-ray binaries
Using a sample of 200 luminous (LX, unabs > 1038 erg s-1, where LX, unabs is the unabsorbed 0.25-8 keV luminosity) high-mass X-ray binary (HMXB) candidates found with Chandra in 27 nearby galaxies, we have constructed the collective X-ray spectrum of HMXBs in the local Universe per unit star formation rate, corrected for observational biases associated with intrinsic diversity of HMXB spectra and X-ray absorption in the interstellar medium. This spectrum is well fit by a power law with a photon index Γ = 2.1 ± 0.1 and is dominated by ultraluminous X-ray sources with LX, unabs > 1039 erg s-1. Hard sources (those with the 0.25-2 to 0.25-8 keV flux ratio of <0.6) dominate above ˜2 keV, while soft and supersoft sources (with the flux ratios of 0.6-0.95 and >0.95, respectively) at lower energies. The derived spectrum probably represents the angle-integrated X-ray emission of the near- and supercritically accreting stellar mass black holes and neutron stars in the local Universe. It provides an important constraint on supercritical accretion models and can be used as a reference spectrum for calculations of the X-ray preheating of the Universe by the first generations of X-ray binaries.