X-Ray Binaries and the Dynamical States of Globular Clusters

TL;DR

Recent research suggests that most Galactic globular clusters are still contracting and have not reached thermal equilibrium, based on X-ray binary abundance, simulation data, and dynamical modeling.

Contribution

This paper synthesizes recent findings indicating that many globular clusters are still in core contraction, challenging previous assumptions of their dynamical state.

Findings

Core-collapsed clusters are overrepresented in X-ray binaries.

Simulations show binary-burning phase matches core-collapsed cluster structures.

Clusters in binary-burning phase have significantly more X-ray sources.

Abstract

We summarize and discuss recent work (Fregeau 2007) that presents the confluence of three results suggesting that most Galactic globular clusters are still in the process of core contraction, and have not yet reached the thermal equilibrium phase driven by binary scattering interactions: that 1) the three clusters that appear to be overabundant in X-ray binaries per unit encounter frequency are observationally classified as "core-collapsed," 2) recent numerical simulations of cluster evolution with primordial binaries show that structural parameters of clusters in the binary-burning phase agree only with "core-collapsed" clusters, and 3) a cluster in the binary-burning phase for the last few Gyr should have about 5 times more dynamically formed X-ray sources than if it were in the core contraction phase for the same time.