The Effect of Distant Encounters on Black Hole Binaries in Globular Clusters: Systematic Increase of In-cluster Mergers in the LISA band

Abstract

We present the first systematic study on how distant weak interactions impact the dynamical evolution of merging binary black holes (BBHs) in dense stellar clusters. Recent studies indicate that dense clusters are likely to significantly contribute to the rate of merging BBHs observable through gravitational waves (GWs), and that many of these mergers will appear with notable eccentricities measurable in the LISA and LIGO sensitivity bands. This is highly interesting, as eccentricity can be used to distinguish between different astrophysical merger channels. However, all of these recent studies are based on various Monte Carlo (MC) techniques that only include strong interactions for the dynamical evolution of BBHs, whereas any binary generally undergoes orders-of-magnitude more weak interactions than strong. It is well known that weak interactions primarily lead to a change in the binary's eccentricity, which for BBHs implies that weak interactions can change their GW inspiral time and thereby their merger probability. With this motivation, we perform MC simulations of BBHs evolving in dense clusters under the influence of both weak and strong interactions. We find that including weak interactions leads to a notable increase in the number of BBHs that merge inside their cluster, which correspondingly leads to a higher number of eccentric LISA sources. These preliminary results illustrate the importance of including weak interactions for accurately modeling how BBHs merge in clusters, and how to link their emitted GW signals to their astrophysical environment.