Using a sample of two hundred luminous ( L _ { { X,unabs } } > 10 ^ { 38 } erg s ^ { -1 } , where L _ { { X,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 \Gamma = 2.1 \pm 0.1 and is dominated by ultraluminous X-ray sources with L _ { { X,unabs } } > 10 ^ { 39 } erg s ^ { -1 } .
Hard sources ( those with the 0.25–2 keV to 0.25–8 keV flux ratio of < 0.6 ) dominate above \sim 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 super-critically 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 .