Spin-up and mass-gain in hyperbolic encounters of spinning black holes

TL;DR

This study uses numerical relativity simulations to analyze how spinning black holes gain mass and spin during hyperbolic encounters, revealing dependencies on initial spins, momenta, and incident angles.

Contribution

It provides new insights into the spin-up and mass-gain processes of black holes during hyperbolic encounters across a wide parameter space.

Findings

Maximum spin-up of 0.3 observed.

Black-hole mass can increase by up to 15%.

Spin-up decreases linearly with initial spin at threshold angles.

Abstract

Scattering black holes spin up and gain mass through the re-absorption of orbital angular momentum and energy radiated in gravitational waves during their encounter. In this work, we perform a series of numerical relativity simulations to investigate the spin-up and mass-gain for equal-mass black holes with a wide range of equal initial spins, $\chi_{\rm i}\in[-0.7,0.7]$, aligned (or anti-aligned) to the orbital angular momentum. We also consider a variety of initial momenta. Furthermore, we explore a range of incident angles and identify the threshold between scattering and merging configurations. The spin-up and mass-gain are typically largest in systems with incident angles close to the threshold value, large momenta, and negative (i.e. anti-aligned) initial spins. When evaluated at the threshold angle, we find that the spin-up decreases linearly with initial spin. Intriguingly, systems with initial spin $\chi_{\rm i}=0.7$ sometimes experience a spin-down, in spite of an increase in the black-hole angular momentum, due to a corresponding gain in the black-hole mass. Across the simulation suite, we find a maximum spin-up of $0.3$ and a maximum increase in the black-hole mass of $15\%$.