| Abstract: | The X-ray appearance of majority of normal galaxies is determined by radiation from accreting neutron stars and stellar mass black holes in X-ray binaries. Chandra and XMM-Newton observations revealed that their populations scale with the star-formation rate and stellar mass of the host galaxy, i.e. X-ray luminosity of a galaxy can be used as a star-formation rate and stellar mass proxy. Luminosity distributions of X-ray binaries obey universal luminosity functions which are qualitatively different in young and old galaxies, reflecting the fundamental difference in the accretion regimes in high- and low-mass X-ray binaries. The luminosity of brightest of them may exceed the Eddington luminosity limit for a solar mass object by ~2-3 orders of magnitudes. The enigma of these sources - dubbed ultra-luminous X-ray sources, still remains largely unsolved. The numbers of high-mass X-ray binaries observed in star-forming galaxies indicate the rather high probability for a massive star to become an accretion powered X-ray source once upon its lifetime. This explains the unexpectedly high contribution of accreting compact objects of stellar mass to the Cosmic X-ray background, ~7-10%, mostly via X-ray emission of faint star-forming galaxies located at moderate redshifts. Their contribution is comparable to the unresolved fraction of the cosmic X-ray background. |
