On the nature of the break in the X-ray luminosity function of low-mass X-ray binaries

We analyze a flux-limited sample of persistent and bright (with 2–10 keV fluxes exceeding 1.4 × 10-10 erg s-1 cm-2) low-mass X-ray binaries (LMXBs) in our Galaxy. It is demonstrated that the majority of binary systems with X-ray luminosities below log L(erg/s) ~ 37.3 have unevolved secondary companions (except for those with white dwarf donors), while systems with higher X-ray luminosity harbor predominantly giant donors. Mass transfer in binary systems with giants significantly shortens their lifetime thus steepening the X-ray luminosity function of LMXBs at high luminosity. We argue that this is why the LMXB luminosity function constructed from observations of sources in our and distant galaxies demonstrates a break at log L(erg/s) ~ 37.3