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

TL;DR

This paper investigates the break in the X-ray luminosity function of low-mass X-ray binaries, linking it to the types of donor stars and their impact on binary evolution, explaining the observed luminosity distribution.

Contribution

It provides a detailed analysis connecting donor star types to the luminosity function break, offering a physical explanation for the observed luminosity distribution in LMXBs.

Findings

Systems with lower luminosity have unevolved secondaries.

Higher luminosity systems mostly have giant donors.

The luminosity function break occurs at logL~37.3 due to donor star evolution.

Abstract

We analyze a flux-limited sample of persistent and bright (with 2-10 keV fluxes exceeding 1.4e-10 erg/s/cm2) low-mass X-ray binaries (LMXBs) in our Galaxy. It is demonstrated that the majority of binary systems with X-ray luminosities below logL(erg/sec)~37.3 have unevolved secondary companions (except for those with white dwarf donors), while systems with higher X-ray luminosity predominantly harbor giant donors. Mass transfer in binary systems with giants significantly shortens their life time thus steepening the X-ray luminosity function of LMXBs at high luminosity. We argue that this is the reason why the LMXB luminosity function constructed in the last years from observations of sources in our and distant galaxies demonstrates a break at logL(erg/sec)~37.3.