Ultraluminous X-ray sources in Globular Clusters

TL;DR

This study explores how dynamical interactions in globular clusters influence the formation, evolution, and ejection of ultraluminous X-ray sources, revealing their potential contribution to field ULX populations.

Contribution

It provides new insights into the role of cluster dynamics and initial conditions in ULX formation and identifies a population of ejected ULXs affecting observations.

Findings

Dynamical processes can both promote and hinder ULX formation.

Non-tidally filling clusters host more ULXs than tidally filling ones.

Ejected ULXs may account for a significant portion of field ULX observations.

Abstract

This paper investigates the formation, populations, and evolutionary paths of UltraLuminous X-ray Sources (ULXs) within Globular Clusters (GCs). ULXs, characterised by their extreme X-ray luminosities, present a challenge to our understanding of accretion physics and compact object formation. While previous studies have largely focused on field populations, this research examines the unique environment of GCs, where dynamical interactions play a significant role. Using the MOCCA Monte Carlo code, we explore how dynamics influences ULX populations within these dense stellar clusters. Our findings reveal that dynamical processes, such as binary hardening and exchanges, can both facilitate and impede ULX formation in GCs. The study explores the impact of parameters including the initial binary fraction, tidal filling, and multiple stellar populations on the evolution of ULXs. We find that non-tidally filling clusters exhibit significantly larger ULX populations compared to tidally filling ones. The results indicate that the apparent scarcity of ULXs in GCs may be related to the older stellar populations of GCs relative to the field. Furthermore, the study identifies a population of "escaper" ULXs, which originate in GCs but are ejected and emit X-rays outside the cluster. These escapers may significantly contribute to the observed field ULX population.