X-Ray Binaries and the Current Dynamical States of Galactic Globular Clusters

TL;DR

This paper refines the understanding of X-ray source abundance in globular clusters by linking it to dynamical evolution, explaining observed anomalies, and suggesting many clusters are still contracting.

Contribution

It introduces a refined encounter frequency model that accounts for X-ray source lifetime and cluster evolution, explaining deviations from previous correlations.

Findings

The refined model explains outliers in the X-ray source versus encounter frequency correlation.

Most globular clusters are still undergoing core contraction and have not reached thermal equilibrium.

The study resolves discrepancies between observed and predicted core radii of clusters.

Abstract

It has been known for over 30 years that Galactic globular clusters (GCs) are overabundant by orders of magnitude in bright X-ray sources per unit mass relative to the disk population. Recently a quantitative understanding of this phenomenon has developed, with a clear correlation between the number of X-ray sources in a cluster, $N_X$, and the cluster's encounter frequency, $\Gamma$, becoming apparent. We derive a refined version of $\Gamma$ that incorporates the finite lifetime of X-ray sources and the dynamical evolution of clusters. With it we find we are able to explain the few clusters that lie off the $N_X$--$\Gamma$ correlation, and resolve the discrepancy between observed GC core radii and the values predicted by theory. Our results suggest that most GCs are still in the process of core contraction and have not yet reached the thermal equilibrium phase driven by binary scattering interactions.