The X-ray luminosity function of low mass X-ray binaries in early-type galaxies, their metal-rich, and metal-poor globular clusters

TL;DR

This study analyzes the X-ray luminosity functions of low mass X-ray binaries in early-type galaxies, revealing differences between those in globular clusters and in the field, with implications for binary populations and metallicity effects.

Contribution

It provides the first detailed comparison of XLFs of LMXBs in GCs and fields across multiple early-type galaxies, highlighting differences related to their environments and metallicity.

Findings

Field LMXB populations are consistent across galaxies regardless of GC specific frequency.

GC LMXBs have a flatter XLF, with more bright sources and fewer faint sources.

Metal-poor GCs may host more bright LMXBs, but not significantly.

Abstract

We present the X-ray luminosity function (XLF) of low mass X-ray binaries (LMXBs) in the globular clusters (GCs) and fields of seven early-types galaxies. These galaxies are selected to have both deep Chandra observations, which allow their LMXB populations to be observed to X-ray luminosities of $10^{37}-10^{38}$ erg/s, and HST optical mosaics which enable the X-ray sources to be separated into field LMXBs, GC LMXBs, and contaminating background and foreground sources. We find that at all luminosities the number of field LMXBs per stellar mass is similar in these galaxies. This suggests that the field LMXB populations in these galaxies are not effected by the GC specific frequency, and that properties such as binary fraction and the stellar initial mass function are either similar across the sample, or change in a way that does not effect the number of LMXBs. We compare the XLF of the field LMXBs to that of the GC LMXBs and find that they are significantly different with a p-value of $3\times10^{-6}$ (equivalent to 4.7$\sigma$ for a normal distribution). The difference is such that the XLF of the GC LMXBs is flatter than that of the field LMXBs, with the GCs hosting relatively more bright sources and fewer faint sources. A comparison of the XLF of the metal-rich and metal-poor GCs hints that the metal-poor GCs may have more bright LMXBs, but the difference is not statistically significant.