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 $\sim 175\mathrm{km}/\mathrm{s}$ can be reached for a mass ratio $\simeq 0.36$. 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 $\sim 440\mathrm{km}/\mathrm{s}$ for black holes having unequal spins that are antialigned and parallel to the orbital angular momentum.

• Received 29 January 2007

DOI:https://doi.org/10.1103/PhysRevLett.99.041102