Resonant recoil in extreme mass ratio binary black hole mergers

TL;DR

This paper discovers a new resonant recoil effect in extreme mass ratio binary black hole mergers, which can dominate the recoil velocity under specific conditions involving high spins and orbital inclinations.

Contribution

It reveals a novel resonance-induced recoil mechanism with a different eta-scaling, challenging previous assumptions about recoil behavior in extreme mass ratio mergers.

Findings

Resonance crossings cause a recoil scaling of V ∝ eta^{3/2}.

Resonant recoil can dominate for high spins and specific orbital inclinations.

The effect suggests caution in extrapolating numerical results to extreme mass ratios.

Abstract

The inspiral and merger of a binary black hole system generally leads to an asymmetric distribution of emitted radiation, and hence a recoil of the remnant black hole directed opposite to the net linear momentum radiated. The recoil velocity is generally largest for comparable mass black holes and particular spin configurations, and approaches zero in the extreme mass ratio limit. It is generally believed that for extreme mass ratios eta<<1, the scaling of the recoil velocity is V {\propto} eta^2, where the proportionality coefficient depends on the spin of the larger hole and the geometry of the system (e.g. orbital inclination). Here we show that for low but nonzero inclination prograde orbits and very rapidly spinning large holes (spin parameter a*>0.9678) the inspiralling binary can pass through resonances where the orbit-averaged radiation-reaction force is nonzero. These resonance crossings lead to a new contribution to the kick, V {\propto} eta^{3/2}. For these configurations and sufficiently extreme mass ratios, this resonant recoil is dominant. While it seems doubtful that the resonant recoil will be astrophysically significant, its existence suggests caution when extrapolating the results of numerical kick results to extreme mass ratios and near-maximal spins.