Superkicks in Hyperbolic Encounters of Binary Black Holes

Abstract

Generic inspirals and mergers of binary black holes produce beamed emission of gravitational radiation that can lead to a gravitational recoil or kick of the final black hole. The kick velocity depends on the mass ratio and spins of the binary as well as on the dynamics of the binary configuration. Studies have focused so far on the most astrophysically relevant configuration of quasi-circular inspirals, for which kicks as large as ∼ 3, 300 km s −1 have been found. We present the first study of gravitational recoil in hyperbolic encounters. Contrary to quasi-circular configurations, in which the beamed radiation tends to average during the inspiral, radiation from hyperbolic encounters is plunge dominated, resulting in an enhancement of preferential beaming. As a consequence, it is possible to achieve kick velocities as large as 10, 000 km s −1 .PACS numbers: 04.60. Kz,04.60.Pp,98.80.Qc Numerical relativity estimates of the gravitational recoil or kick inflicted on the final black hole (BH) from generic inspirals and mergers of binary black holes (BBH) have triggered tremendous excitement in astrophysics. This is mainly due to the fact that most galaxies host a supermassive black hole (SMBH) at their centers [1, 2]. As galaxies merge, a kick to the final BH from the coalescence of the BHs at the galactic cores could have profound implications in subsequent mergers, affecting the growth of SMBHs via mergers as well as the population of galaxies containing SMBHs. In addition, there have been several suggestions of direct observational signatures of putative BH recoils [3,4,5,6,7,8], with one study [9] presenting evidence for the first candidate of a recoiling SMBH.BH kick velocities depend on the mass ratio and spins of the merging BHs as well as on the initial configuration and subsequent dynamics of the binary. Studies published to date have concerned the most astrophysically relevant configuration, that of quasi-circular inspirals [10,11,12,13]. One remarkable discovery has been kicks of a few thousand km s −1 found in configurations of equal-mass binaries with initially anti-aligned spins in the orbital plane [14,15,16]. For near extremal spins (a/m = 0.925), recoils as large as 3, 300 km s −1 have been computed [17].Motivated by our previous study [18] of the final spin of BHs from scattering mergers of BBHs, we present the first extension of gravitational recoil to hyperbolic encounters. There are crucial differences between hyperbolic and quasi-circular configurations that affect the kick to the final BH. For quasi-circular orbits, the emitted radiation is asymmetrically beamed, and carries linear momentum with it. But it tends to average out during the inspiral [19], producing a modest wobbling and drift of the center of mass of the binary. The final kick arises because as the binary approaches the plunge, the averaging loses its effectiveness, leading to a gradual recoil build-up. Both numerical simulations and postNewtonian studies [19,20] have confirmed the gradual kick accumulation during the inspir...

MYRIAD: a newN-body code for simulations of star clusters

“…According to numerical relativity results, when two spinning black holes collide, gravitational radiation could be emitted asymmetrically. This would lead to a recoil velocity in the resulting black hole that might be as high as 4000 km s −1 (Gonzalez et al 2007;Campanelli et al 2007b,a;Healy et al 2009;Herrmann et al 2007), depending on the mass ratio of the initial black holes and the directions of their spins, but this velocity might be significantly suppressed by the relativistic alignment of the spins (Kesden et al 2010). It is not easy, of course to include numerical relativity in an N-body code, but semi-analytical formulae, coming from fitting between numerical relativity results and post-Newtonian theory (Lousto & Zlochower 2008;Baker et al 2008;Lousto et al 2010) to determine the direction and magnitude of the recoil velocity.…”

Black-hole binary simulations: The mass ratio10∶1

“…For example, numerical relativity has provided vital information regarding the kick or recoil in astrophysical mergers [14,15,16,17,18,19,20,21,22,23], simulations of spin flip and precession phenomena [15,24] and the interpretation of gravitational waveforms to be observed in the future [25,26,27,28,29,30]. See also [31,32,33,34,35] for binary simulations involving neutron stars.…”

Status of NINJA: the Numerical INJection Analysis project

“…Following the pioneer work of Pretorius [23], and the independent results from the Goddard [24] and Brownsville (now at RIT) [25] groups, several numerical relativity groups around the world have successfully evolved various BBH configurations. This has lead to important new physical insights, such as the prediction of large recoil velocities due to asymmetric emission of gravitational radiation during the merger [26][27][28][29][30][31][32][33][34][35][36][37][38][39][40][41][42] and the prediction of the parameters of the remnant for a wide class of initial states [43][44][45][46][47][48][49][50][51][52][53][54]. A review of the current status of BBH simulations is available in this volume [55].…”