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

¹ Excellence Cluster Universe, Technische Universität München, Boltzmannstr. 2, 85748 Garching, Germany
e-mail: mikej@mpa-garching.mpg.de
² Space Research Institute, Russian Academy of Sciences, Profsoyuznaya 84/32, 117997 Moscow, Russia
³ Sternberg Astronomical Institute, Moscow State University, Universitetskij pr., 13, 119992 Moscow, Russia
⁴ Max-Planck-Institute für Astrophysik, Karl-Schwarzschild-Str. 1, 85740 Garching bei München, Germany

Received: 2 April 2010
Accepted: 18 November 2010

Abstract

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.