X-ray binaries and ultra-luminous X-ray sources in nearby and distant galaxies

Abstract

We review recent results on populations of compact X-ray sources in normal galaxies. The luminosity distributions of low and high mass X-ray binaries in nearby galaxies appear to be described by the respective ``universal'' luminosity functions, whose shapes do not vary significantly from galaxy to galaxy and the normalizations are proportional to the SFR (HMXBs) and stellar mass (LMXBs) of the host galaxy. There is a significant qualitative difference between XLFs of high and low mass X-ray binaries, reflecting the difference in the accretion regimes in these two types of X-ray binaries. The high luminosity cut-off is by an order of magnitude larger for HMXBs -- although bright sources, log(Lx)>39, are observed in both young and old galaxies, the truly ultraluminous ones, with log(Lx)~40-40.5, are associated with the regions of intense star formation and have not been detected so far in old stellar populations of elliptical and S0 galaxies. The Lx-SFR relation for distant galaxies in the HDF-N indicates that ULXs at redshifts of z~0.2-1.3 were not significantly more luminous than those observed in nearby galaxies.