Recoil Velocities from Equal-Mass Binary-Black-Hole Mergers

The final evolution of a binary-black-hole system gives rise to a recoil velocity if an asymmetry is present in the emitted gravitational radiation. Measurements of this effect for nonspinning binaries with unequal masses have pointed out that kick velocities ∼175  km/s can be reached for a mass ratio ≃0.36. However, a larger recoil can be obtained for equal-mass binaries if the asymmetry is provided by the spins. Using two independent methods we show that the merger of such binaries yields velocities as large as ∼440  km/s for black holes having unequal spins that are antialigned and parallel to the orbital angular momentum.