On the origin of the metallicity dependence in dynamically formed extragalactic low-mass X-ray binaries

Abstract

Globular clusters (GCs) effectively produce dynamically-formed low-mass X-ray binaries (LMXBs). Observers detect ~100 times more LMXBs per stellar mass in GCs compared to stars in the fields of galaxies. It has also been observationally established that metal-rich GCs are about 3 times more likely to contain an X-ray source than their metal-poor counterparts. Recent observations have shown that this ratio holds in extragalactic GCs for all bright X-ray sources with Lx between 2X10^{37} and 5X10^{38} erg/s. In this Letter, we propose that the observed metallicity dependence of LMXBs in extragalactic GCs can be explained by the differences in the number densities and average masses of red giants in populations of different metallicities. Red giants serve as seeds for the dynamical production of bright LMXBs via two channels - binary exchanges and physical collisions - and the increase of the number densities and masses of red giants boost LMXB production, leading to the observed difference. We also discuss a possible effect of the age difference in stellar populations of different metallicities.

5 pages, 2 figures, submitted to ApJL

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Cited by in corpus (10)

• Metallicity Effect on LMXB Formation in Globular Clusters
• The Two-Dimensional Spatial Distributions of the Globular Clusters and Low-Mass X-Ray Binaries of NGC4649
• The radial distribution of X-ray Binaries and Globular Clusters in NGC 4649 and their relation with the local stellar mass density
• The X-ray Emissivity of Low-Density Stellar Populations
• The Stellar Age Dependence of X-ray Emission from Normal Star-Forming Galaxies in the GOODS Fields
• The Luminosity Function of Low Mass X-Ray Binaries in the Globular Cluster System of NGC 1399
• M31 globular cluster structures and the presence of X-ray binaries
• Milky Way Globular Cluster Metallicity and Low-Mass X-ray Binaries: The Red Giant Influence
• Evolution of binaries with compact objects in globular clusters
• An excess of low-mass X-ray binaries in the outer halo of NGC 4472