Ultraluminous X-ray sources in Globular Clusters

¹ Nicolaus Copernicus Astronomical Center, Polish Academy of Sciences, Bartycka 18, 00-716 Warsaw, Poland
² Faculty of Mathematics and Computer Science, A. Mickiewicz University, Uniwersytetu Poznańskiego 4, 61-614 Poznań, Poland

^★ Corresponding author; gwiktoro@camk.edu.pl

Received: 10 January 2025
Accepted: 5 March 2025

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. Our simulations reveal that these escapers constitute about one-seventh of the total ULX population. However, for neutron star accretors specifically, escapers are twice as common as their in-cluster counterparts. Notably, only 4% of in-cluster ULXs contain neutron star accretors. These escapers may significantly contribute to the observed field ULX population.