Large Merger Recoils and Spin Flips From Generic Black-Hole Binaries
Manuela Campanelli, Carlos O. Lousto, Yosef Zlochower, David Merritt
TL;DR
This study investigates large merger recoils and spin flips in generic unequal-mass black-hole binaries using fully non-linear numerical relativity. By simulating a $q \approx 1.99$ system with a highly spinning primary ($a/m=0.885$) misaligned by $-45^\circ$ and a negligible-spin companion, they observe a spin-induced recoil of $V_{\rm recoil} \approx 454$ km s$^{-1}$ and a remnant-spin flip of $\approx 103^\circ$, along with substantial spin precession. A second SP2 run with in-plane anti-aligned spins yields $V_{\rm recoil} \approx 1830$ km s$^{-1}$, scalable to $\sim 4000$ km s$^{-1}$ for maximal spins, illustrating that spin effects can dominate recoil. The authors propose an empirical recoil model $\vec{V}_{\rm recoil}(q,\vec{\alpha})$ and discuss astrophysical implications, including possible ejection of SMBHs from galactic nuclei and jet-direction changes, with potential relevance to X-shaped radio sources.
Abstract
We report the first results from evolutions of a generic black-hole binary, i.e. a binary containing unequal mass black holes with misaligned spins. Our configuration, which has a mass ratio of 2:1, consists of an initially non-spinning hole orbiting a larger, rapidly spinning hole (specific spin a/m = 0.885), with the spin direction oriented -45 degrees with respect to the orbital plane. We track the inspiral and merger for ~2 orbits and find that the remnant receives a substantial kick of 454 km/s, more than twice as large as the maximum kick from non-spinning binaries. The remnant spin direction is flipped by 103 degrees with respect to the initial spin direction of the larger hole. We performed a second run with anti-aligned spins, a/m = +-0.5 lying in the orbital plane that produces a kick of 1830 km/s off the orbital plane. This value scales to nearly 4000 km/s for maximally spinning holes. Such a large recoil velocity opens the possibility that a merged binary can be ejected even from the nucleus of a massive host galaxy.