Dynamics of massive stellar black holes in young star clusters and the displacement of ultra-luminous X-ray sources

TL;DR

This study uses simulations to explore how massive stellar black holes in young star clusters are ejected and how their interactions influence their properties, shedding light on the origins of ultra-luminous X-ray sources.

Contribution

It provides the first detailed simulation-based analysis of MSBH dynamics in young clusters, including ejection mechanisms and binary property evolution.

Findings

Approximately 44% of MSBH binaries are ejected within 10 Myr.

Ejected MSBHs have offsets consistent with observed X-ray sources.

Three-body encounters increase binary eccentricity and alter semi-major axes.

Abstract

In low-metallicity environments, massive stars might avoid supernova explosion and directly collapse, forming massive (~25-80 solar masses) stellar black holes (MSBHs), at the end of their life. MSBHs, when hosted in young massive clusters, are expected to form binaries and to strongly interact with stars, mainly via three-body encounters. We simulate various realizations of young star clusters hosting MSBHs in hard binaries with massive stars. We show that a large fraction (~44 per cent) of MSBH binaries are ejected on a short timescale (<=10 Myr). The offset of the ejected MSBHs with respect to the parent cluster is consistent with observations of X-ray binaries and ultra-luminous X-ray sources. Furthermore, three-body encounters change the properties of MSBH binaries: the semi-major axis changes by <=50 per cent and the eccentricity of the system generally increases. We shortly discuss the implications of our simulations on the formation of high-mass X-ray binaries hosting MSBHs.