Interactions among binary black holes in star clusters: Eccentric gravitational wave captures and triple formation

¹ Departament de Física Quàntica i Astrofísica (FQA), Universitat de Barcelona (UB), Martí i Franquès 1, 08028 Barcelona, Spain
² Institut de Ciències del Cosmos (ICCUB), Universitat de Barcelona (UB), Martí i Franquès 1, 08028 Barcelona, Spain
³ ICREA, Pg. Lluís Companys 23, 08010 Barcelona, Spain
⁴ Niels Bohr International Academy, Niels Bohr Institute, Blegdamsvej 17, 2100 Copenhagen, Denmark

^★ Corresponding author: danielmarin@icc.ub.edu

Received: 19 December 2024
Accepted: 6 May 2025

Abstract

Aims. Numerical simulations of star clusters with black holes find that there is only a single dynamically active binary black hole (BBH), at odds with the theoretical expectation of ∼5 dynamically formed ‒ or, commonly referred to as ‘three-body’ ‒ BBHs in clusters with a few hundred BHs. We test the recent suggestion that this tension is because interactions among three-body BBHs were neglected in the theory.

Methods. We use the public catalogue of Cluster Monte Carlo models to obtain a sample of strong BBH-BBH interactions, which we integrate using post-Newtonian equations of motion up to 3.5 PN. We explore the nature of the BBHs involved in BBH − BBH interactions in star clusters, as well as the various outcomes: gravitational wave (GW) captures and the associated eccentricities at the frequencies of ground-based GW detectors, as well as BH triple formation and their contribution to BBH mergers via the Lidov-Kozai mechanism.

Results. We find that almost all BBHs involved in BBH − BBH interactions are indeed three-body binaries and that BBH formation and disruption in BBH − BBH interactions occur at approximately the same rate, providing an explanation for the finding of a single dynamically active BBH in N-body models. An important implication is that the resulting rates of GW capture and triple formation are independent of uncertain initial binary properties. With the use of a population synthesis model for BBH − BBH interactions in globular clusters, we obtain a local rate of GW captures of ℛ(z ≃ 0) ≃ 1 Gpc⁻³ yr⁻¹, as well as their eccentricity distribution and redshift dependence. We find that a BBH − BBH interaction is more likely to trigger a GW merger than a BH − BBH interaction. We also confirm that stable triples that are assembled in BBH − BBH interactions can merge via von Zeipel-Lidov-Kozai oscillations, although their merger rate is lower than GW captures. Our results will help with the interpretation of future GW signals from eccentric BBHs.