The Effect of Starburst Metallicity on Bright X-Ray Binary Formation Pathways

TL;DR

This study explores how the metallicity of starburst environments influences the formation pathways and population characteristics of young, bright high-mass X-ray binaries, highlighting the dominant formation mechanisms at different metallicities.

Contribution

It introduces a detailed analysis of metallicity-dependent formation pathways of HMXBs, emphasizing the impact on population numbers rather than individual system parameters.

Findings

Metallicity primarily affects the number of systems in each formation pathway.

Ultra-luminous X-ray sources can be explained by bright HMXBs with stable Roche Lobe overflow.

The HMXB population in NGC 1569 is likely dominated by a very metal-poor starburst cluster.

Abstract

We investigate the characteristics of young (< 20 Myr) and bright (Lx > 1e36 erg/s) High-Mass X-ray Binaries (HMXBs) and find the population to be strongly metallicity-dependent. We separate the model populations among two distinct formation pathways: (1) systems undergoing active Roche Lobe Overflow (RLO), and (2) wind accretion systems with donors in the (super)giant (SG) stage, which we find to dominate the HMXB population. We find metallicity to primarily affect the number of systems which move through each formation pathway, rather than the observable parameters of systems which move through each individual pathway. We discuss the most important model parameters affecting the HMXB population at both low and high metallicities. Using these results, we show that (1) the population of ultra-luminous X-Ray sources can be consistently described by very bright HMXBs which undergo stable Roche Lobe overflow with mild super-Eddington accretion and (2) the HMXB population of the bright starburst galaxy NGC~1569 is likely dominated by one extremely metal-poor starburst cluster.