Anomalies in the gravitational recoil of eccentric black-hole mergers with unequal mass ratios

TL;DR

This paper investigates how nonzero eccentricity affects recoil velocities and gravitational-wave emission in nonspinning, unequal-mass black hole mergers, revealing up to 25% increases and oscillatory behavior in kick velocities.

Contribution

It provides the first detailed analysis of eccentricity effects on recoil in unequal-mass, nonspinning black hole mergers, expanding understanding beyond equal-mass, spinning cases.

Findings

Eccentricity can increase recoil velocities by up to 25%.

Kick velocity exhibits oscillatory dependence on eccentricity.

Eccentric mergers produce distinct gravitational-wave signatures.

Abstract

The radiation of linear momentum imparts a recoil (or "kick") to the center of mass of a merging black hole binary system. Recent numerical relativity calculations have shown that eccentricity can lead to an approximate 25% increase in recoil velocities for equal-mass, spinning binaries with spins lying in the orbital plane ("superkick" configurations) [U Sperhake et al. Phys. Rev. D 101 (2020) 024044 (arXiv:1910.01598)]. Here we investigate the impact of nonzero eccentricity on the kick magnitude and gravitational-wave emission of nonspinning, unequal-mass black hole binaries. We confirm that nonzero eccentricities at merger can lead to kicks which are larger by up to ~25% relative to the quasicircular case. We also find that the kick velocity $v$ has an oscillatory dependence on eccentricity, that we interpret as a consequence of changes in the angle between the infall direction at merger and the apoapsis (or periapsis) direction.