Anatomy of the binary black hole recoil: A multipolar analysis

TL;DR

This paper provides a detailed multipolar analysis of gravitational recoil in binary black hole mergers, demonstrating that a limited set of modes accurately predicts recoil velocities and explaining the underlying physics of the recoil process.

Contribution

It introduces a multipolar framework up to l=4 modes that accurately models recoil velocities and phases, linking numerical results with analytic Kerr QNM formulas.

Findings

Multipole moments up to l=4 suffice for ~2% recoil accuracy.

A few dominant modes control recoil build-up.

Analytic Kerr QNM formulas explain anti-kick and recoil differences.

Abstract

We present a multipolar analysis of the gravitational recoil computed in recent numerical simulations of binary black hole (BH) coalescence, for both unequal masses and non-zero, non-precessing spins. We show that multipole moments up to and including l=4 are sufficient to accurately reproduce the final recoil velocity (within ~2%) and that only a few dominant modes contribute significantly to it (within ~5%). We describe how the relative amplitudes, and more importantly, the relative phases, of these few modes control the way in which the recoil builds up throughout the inspiral, merger, and ringdown phases. We also find that the numerical results can be reproduced by an ``effective Newtonian'' formula for the multipole moments obtained by replacing the radial separation in the Newtonian formulae with an effective radius computed from the numerical data. Beyond the merger, the numerical results are reproduced by a superposition of three Kerr quasi-normal modes (QNMs). Analytic formulae, obtained by expressing the multipole moments in terms of the fundamental QNMs of a Kerr BH, are able to explain the onset and amount of ``anti-kick'' for each of the simulations. Lastly, we apply this multipolar analysis to help explain the remarkable difference between the amplitudes of planar and non-planar kicks for equal-mass spinning black holes.