The missing link between ultraluminous X-ray sources and metallicity

TL;DR

This paper investigates how low metallicity environments contribute to the formation of massive stellar black holes, which can explain the nature of ultraluminous X-ray sources without exceeding the Eddington limit.

Contribution

It introduces N-body simulations with metallicity-dependent stellar evolution to explore pathways for forming X-ray binaries with massive stellar black holes.

Findings

Anti-correlation between metallicity and ULX numbers

MSBHs can account for ULXs without super-Eddington accretion

Simulations identify key formation pathways for MSBH-powered X-ray binaries

Abstract

The nature of ultraluminous X-ray sources (ULXs) is still debated. Recent studies show that metal-poor massive stars can collapse into massive stellar black holes (MSBHs), that is black holes with mass > 25 Msun. Such MSBHs are sufficiently massive to explain most ULXs without requiring substantial violations of the Eddington limit. The recent finding of an anti-correlation between metallicity of the environment and number of ULXs per galaxy supports this hypothesis. We present the results of recent N-body simulations, including metallicity dependent stellar evolution, and we discuss the main pathways to produce X-ray binaries powered by MSBHs.