Superkicks in ultrarelativistic encounters of spinning black holes

TL;DR

This study uses numerical relativity simulations to analyze ultrarelativistic encounters of spinning black holes, revealing that superkicks can exceed 15,000 km/s and are mainly caused by asymmetric gravitational wave emission due to orbital bobbing.

Contribution

The paper demonstrates that superkicks in black hole encounters are largely independent of merger occurrence and are driven by orbital dynamics, providing new insights into recoil mechanisms.

Findings

Superkicks can exceed 15,000 km/s in ultrarelativistic encounters.

Maximum recoil correlates with gravitational wave energy radiated.

Recoil mechanism is primarily due to asymmetric radiation from orbital bobbing.

Abstract

We study ultrarelativistic encounters of two spinning, equal-mass black holes through simulations in full numerical relativity. Two initial data sequences are studied in detail: one that leads to scattering and one that leads to a grazing collision and merger. In all cases, the initial black hole spins lie in the orbital plane, a configuration that leads to the so-called "superkicks". In astrophysical, quasicircular inspirals, such kicks can be as large as ~3,000 km/s; here, we find configurations that exceed ~15,000 km/s. We find that the maximum recoil is to a good approximation proportional to the total amount of energy radiated in gravitational waves, but largely independent of whether a merger occurs or not. This shows that the mechanism predominantly responsible for the superkick is not related to merger dynamics. Rather, a consistent explanation is that the "bobbing" motion of the orbit causes an asymmetric beaming of the radiation produced by the in-plane orbital motion of the binary, and the net asymmetry is balanced by a recoil. We use our results to formulate some conjectures on the ultimate kick achievable in any black hole encounter.