Gravitational Wave Capture in Spinning Black Hole Encounters

TL;DR

This paper investigates how the spins of black holes affect gravitational wave captures during close encounters, revealing that spin alignment influences energy emission and merging likelihood in star clusters.

Contribution

It provides the first numerical relativity analysis of spin effects on gravitational wave capture and merging in black hole encounters.

Findings

Energy radiated depends on effective spin, with maximum emission for anti-aligned spins in fly-by encounters.

Aligned spins produce the strongest emission during direct mergers.

Approximately 5% of GW captures result in direct merging at a velocity dispersion of 150 km/s.

Abstract

The orbits of two black holes which are initially unbound can be transformed into bound orbits by emitting gravitational waves during close encounters in a star cluster, which is called a gravitational wave (GW) capture. The effects of spin of black holes on GW capture are investigated in the context of numerical relativity. The radiated energy during the encounter is dependent on the effective spin when the black holes have the equal masses as expected from post-Newtonian approximation. The strongest emission is produced when the spins of both black holes are anti-aligned to the orbital angular momentum in the case of fly-by encounters. But the opposite is true in the case of direct merging: the strongest emission comes from the black holes with aligned spins to the orbital angular momentum. The fraction of direct merging among the GW captures increases in proportional to $v^{4/7}$ assuming the uniform distribution of pericenter distances in the encounters, where $v$ is the velocity dispersion of cluster, which means about 5 % of GW capture leads to the direct merging for star clusters with $v=150$ km s$^{-1}$.