Exploring black hole superkicks

TL;DR

This paper investigates the physics of black-hole superkicks by analyzing the role of spin and gravitational wave modes in symmetric configurations, revealing proportionality between recoil and specific wave modes and timing of the recoil relative to merger.

Contribution

The study provides new insights into the dynamics of black-hole recoil, especially the proportionality to specific gravitational wave modes and the timing of the recoil around the merger.

Findings

Recoil velocity is proportional to the difference in specific gravitational wave modes.

Major recoil occurs within 30M of the merger time.

Asymmetry in gravitational wave modes affects signal detection and analysis.

Abstract

Recent calculations of the recoil velocity in black-hole binary mergers have found kick velocities of $\approx2500 $km/s for equal-mass binaries with anti-aligned initial spins in the orbital plane. In general the dynamics of spinning black holes can be extremely complicated and are difficult to analyze and understand. In contrast, the ``superkick'' configuration is an example with a high degree of symmetry that also exhibits exciting physics. We exploit the simplicity of this ``test case'' to study more closely the role of spin in black-hole recoil and find that: the recoil is with good accuracy proportional to the difference between the $(l = 2, m = \pm 2)$ modes of $\Psi_4$, the major contribution to the recoil occurs within $30M$ before and after the merger, and that this is after the time at which a standard post-Newtonian treatment breaks down. We also discuss consequences of the $(l = 2, m = \pm 2)$ asymmetry in the gravitational wave signal for the angular dependence of the SNR and the mismatch of the gravitational wave signals corresponding to the north and south poles.