# Black hole and neutron star mergers in Galactic Nuclei: the role of triples

## TL;DR

This study explores how triple systems of black holes and neutron stars near supermassive black holes in galactic nuclei can significantly enhance merger rates, potentially explaining observed gravitational wave events.

## Contribution

It introduces a new scenario involving triple compact objects in galactic nuclei, showing they can increase merger rates compared to binary systems.

## Key findings

- Merger fraction up to 5-8 times higher in triples than binaries.
- Triples can merge on shorter timescales due to dynamical interactions.
- Triples could contribute significantly to LIGO/VIRGO gravitational wave detections.

## Abstract

Nuclear star clusters that surround supermassive black holes (SMBHs) in galactic nuclei are thought to contain large numbers of black holes (BHs) and neutron stars (NSs), a fraction of which form binaries and could merge by Kozai-Lidov oscillations (KL). Triple compact objects are likely to be present, given what is known about the multiplicity of massive stars, whose life ends either as a NS or a BH. In this paper, we present a new possible scenario for merging BHs and NSs in galactic nuclei. We study the evolution of a triple black hole (BH) or neutron star (NS) system orbiting an SMBH in a galactic nucleus by means of direct high-precision $N$-body simulations, including Post-Newtonian terms. We find that the four-body dynamical interactions can increase the KL angle window for mergers compared to the binary case and make BH and NS binaries merge on shorter timescales. We show that the merger fraction can be up to $\sim 5$--$8$ times higher for triples than for binaries. Therefore, even if the triple fraction is only $\sim 10\%$--$20\%$ of the binary fraction, they could contribute to the merger events observed by LIGO/VIRGO in comparable numbers.