Exploring black hole superkicks

Recent calculations of the recoil velocity in black-hole binary mergers have found kick velocities of ≈2500  km/s for equal-mass binaries with antialigned 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=±2) modes of Ψ₄, 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=±2) asymmetry in the gravitational wave signal for the angular dependence of the signal-to-noise ratio and the mismatch of the gravitational wave signals corresponding to the north and south poles.