Gravitational Recoil of Inspiralling Black-Hole Binaries to Second Post-Newtonian Order

The loss of linear momentum by gravitational radiation and the resulting gravitational recoil of black-hole binary systems may play an important role in the growth of massive black holes in early galaxies. We calculate the gravitational recoil of nonspinning black-hole binaries at the second post-Newtonian order (2PN) beyond the dominant effect, obtaining, for the first time, the 1.5PN correction term due to tails of waves and the next 2PN term. We find that the maximum value of the net recoil experienced by the binary due to the inspiral phase up to the innermost stable circular orbit (ISCO) is of the order of 22 km s. We then estimate the kick velocity accumulated during the plunge from the ISCO up to the horizon by integrating the momentum flux using the 2PN formula along a plunge geodesic of the Schwarzschild metric. We find that the contribution of the plunge dominates over that of the inspiral. For a mass ratio m2/m1 = 1/8, we estimate a total recoil velocity (due to both adiabatic and plunge phases) of 100 ± 20 km s−1. For a ratio 0.38, the recoil is maximum and we estimate it to be 250 ± 50 km s−1. In the limit of small mass ratio, we estimate V/c ≈ 0.043 (1± 20%) (m2/m1). Our estimates are consistent with, but span a substantially narrower range than, those of Favata et al. (2004). On leave from University of Sana, Yemen Visiting Researcher at GRεCO – Gravitation et Cosmologie, Institut d’Astrophysique de Paris, C.N.R.S., 98 bis boulevard Arago, 75014 Paris, France