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

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.
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