Recoiling from a kick in the head-on collision of spinning black holes

TL;DR

This paper investigates gravitational recoil kicks from black hole collisions using head-on simulations, revealing significant spin effects and a correlation with post-Newtonian predictions, aiding understanding of black hole merger dynamics.

Contribution

It introduces a simplified head-on collision model to analyze recoil effects, highlighting the importance of spin and mass ratio, and demonstrating consistency with post-Newtonian scaling.

Findings

Spin-induced recoils are significant even in head-on collisions.

Transverse kick scaling with spins aligns with post-Newtonian theory.

Heuristic models may effectively estimate spin-kicks.

Abstract

Recoil ``kicks'' induced by gravitational radiation are expected in the inspiral and merger of black holes. Recently the numerical relativity community has begun to measure the significant kicks found when both unequal masses and spins are considered. Because understanding the cause and magnitude of each component of this kick may be complicated in inspiral simulations, we consider these effects in the context of a simple test problem. We study recoils from collisions of binaries with initially head-on trajectories, starting with the simplest case of equal masses with no spin and then adding spin and varying the mass ratio, both separately and jointly. We find spin-induced recoils to be significant relative to unequal-mass recoils even in head-on configurations. Additionally, it appears that the scaling of transverse kicks with spins is consistent with post-Newtonian theory, even though the kick is generated in the nonlinear merger interaction, where post-Newtonian theory should not apply. This suggests that a simple heuristic description might be effective in the estimation of spin-kicks.