Explaining superkicks: how individual black-hole spins affect merging black-hole binaries

One of the biggest surprises that followed the breakthroughs in numerical relativity beginning in 2005 was the finding that, for certain orientations of the individual black-hole spins before merger, the final merged remnant can receive a "kick" as large as ~5000 km/s. Subsequent efforts to explain this phenomenon have rested on intuition built from the post-Newtonian (PN) approximation. Most of this work has focused on the "radiation rocket" effect of asymmetric gravitational-wave emission due to spin-orbit and spin-spin couplings, although the exchange of momentum between the individual black holes and the surrounding gravitational field has also been explored. However, we show that neither account is correct. We consider direct spin-radiation coupling as the mechanism behind superkicks, where the frame-dragging effect of the individual black-hole spins on the emitted gravitational radiation causes the merging body to recoil. The effect we describe is physically distinct and of a higher PN order than the leading-order spin-orbit interactions that motivated the functional form for kick fitting formulae. We consider a simple model motivated by the Backwards One-Body paradigm, where the merger is treated as a combination of the merged remnant and radiation orbiting around its light ring. We find that this model can fully account for the observed dependence of the recoil on the spin orientations and the phase at merger, including fully account for the "hangup kick" effect.