Study on the detectability of gravitational radiation from single-binary encounters between black holes in nuclear star cluster: the case of hyperbolic flybys

TL;DR

This study investigates the frequency and detectability of hyperbolic gravitational-wave signals from single-binary black hole encounters in nuclear star clusters, finding such events are rare and unlikely to be observed with current detectors.

Contribution

It provides the first analysis of hyperbolic encounter rates in NSCs and assesses their potential observability by GW detectors, focusing on non-resonant interactions.

Findings

Single-binary encounters are rarer than binary mergers in NSCs.

Hyperbolic encounters produce GW signals mostly below terrestrial detector sensitivity.

Some encounters may be detectable by LISA, but detection probability is low.

Abstract

With the release of the third Gravitational-Wave Transient Catalogue (GWTC-3), 90 observations of compact-binary mergers by Virgo and LIGO detectors are confirmed. Some of these mergers are suspected to have occurred in star clusters. The density of black holes at the cores of these clusters is so high that mergers can occur through a few generations forming increasingly massive black holes. These conditions also make it possible for three black holes to interact, most likely via single-binary encounters. In this paper, we present a first study of how often such encounters can happen in nuclear star clusters (NSCs) as a function of redshift, and whether these encounters are observable by gravitational-wave (GW) detectors. This study focuses on effectively hyperbolic encounters leaving out the resonant encounters. We find that in NSCs single-binary encounters occur rarely compared to binary mergers, and that hyperbolic encounters most likely produce the strongest GW emission below the observation band of terrestrial GW detectors. While several of them can be expected to occur per year with peak energy in the LISA band, their amplitude is low, and detection by LISA seems improbable.