Compact Binaries Ejected from Globular Clusters as GW Sources

TL;DR

This study uses N-body simulations to analyze the formation, ejection, and merger rates of compact binaries from globular clusters, highlighting their significance as gravitational wave sources and potential gamma-ray burst progenitors.

Contribution

It provides the first detailed estimates of merger rates for ejected BH-BH and NS-NS binaries from globular clusters based on dynamical simulations.

Findings

30% of ejected compact objects are in binaries.

Merger timescales can be shorter than the universe's age.

Estimated detection rates are 15 and 0.024 per year for BH-BH and NS-NS binaries.

Abstract

Performing N-body simulations, we examine the dynamics of BH-BH (10 Msun each) and NS-NS (1.4 Msun each) binaries formed in a cluster and its implications for gravitational wave detection. A significant fraction of compact binaries are ejected from a globular cluster after core collapse. Among the total number of ejected compact objects, 30 per cent of them are in binaries. Merging time-scales of ejected binaries, which depend on the cluster's velocity dispersion, are in some cases shorter than the age of the universe. During the merging event, these dynamically formed compact mergers are expected to produce gravitational waves that can be detectable by the advanced ground-based interferometers. Based on our reference assumptions, merger rates of ejected BH-BH and NS-NS binaries per globular cluster are estimated to be 2.5 and 0.27 per Gyr, respectively. Assuming the spatial density of globular clusters to be 8.4 h^3 clusters Mpc^-3 and extrapolating the merger rate estimates to the horizon distance of the advanced LIGO-Virgo network, we expect the detection rates for BH-BH and NS-NS binaries with cluster origin are to be 15 and 0.024 yr^-1, respectively. We find out that some of the dynamically formed binaries are ejected with a large escape velocity. They can be responsible for short gamma-ray bursts whose locations are far from host galaxies.