How Black Holes Get Their Kicks: Gravitational Radiation Recoil Revisited

Gravitational waves from the coalescence of binary black holes carry away linear momentum, causing center of mass recoil. This "radiation rocket'' effect has important implications for systems with escape speeds of order the recoil velocity. Previous analyses have suggested that the recoil could exceed several thousand km/s. We revisit this problem using black hole perturbation theory, treating the binary as a test mass spiraling into a spinning hole. For extreme mass ratios (q = m₁/m₂ << 1) we compute the recoil for the slow inspiral epoch of binary coalescence very accurately; these results can be extrapolated to q ~ 0. 4 with modest accuracy. Although the recoil from the final plunge contributes significantly to the final recoil, we are only able to make crude estimates of its magnitude. We find that the recoil can easily reach ~ 100 - 200 km/s, but most likely does not exceed ~ 500 - 600 km/s. Though much lower than previous estimates, this recoil is large enough to have important astrophysical consequences. These include the ejection of black holes from globular clusters, dwarf galaxies, and high-redshift dark matter halos.