Luminosity functions of LMXBs in Centaurus A: globular clusters versus the field

TL;DR

This study analyzes the X-ray luminosity functions of low mass X-ray binaries in Centaurus A, revealing differences between globular cluster and field populations and suggesting a role for helium accreting systems in globular clusters.

Contribution

It provides the first deep XLF analysis of LMXBs in Centaurus A, showing a significant difference between globular cluster and field populations and proposing a new explanation involving helium accretion.

Findings

Confirmed low luminosity break in LMXB XLF at log(L_X)=37.2-37.6

Found a significant underabundance of faint sources in globular clusters

Suggested helium accreting systems explain XLF differences

Abstract

We study the X-ray luminosity function (XLF) of low mass X-ray binaries (LMXB) in the nearby early-type galaxy Centaurus A, concentrating primarily on two aspects of binary populations: the XLF behavior at the low luminosity limit and comparison between globular cluster and field sources. The 800 ksec exposure of the deep Chandra VLP program allows us to reach a limiting luminosity of 8e35 erg/s, about 2-3 times deeper than previous investigations. We confirm the presence of the low luminosity break in the overall LMXB XLF at log(L_X)=37.2-37.6 below which the luminosity distribution follows a constant dN/d(ln L). Separating globular cluster and field sources, we find a statistically significant difference between the two luminosity distributions with a relative underabundance of faint sources in the globular cluster population. This demonstrates that the samples are drawn from distinct parent populations and may disprove the hypothesis that the entire LMXB population in early type galaxies is created dynamically in globular clusters. As a plausible explanation for this difference in the XLFs, we suggest that there is an enhanced fraction of helium accreting systems in globular clusters, which are created in collisions between red giants and neutron stars. Due to the 4 times higher ionization temperature of He, such systems are subject to accretion disk instabilities at approximately 20 times higher mass accretion rate, and therefore are not observed as persistent sources at low luminosities.