  [abridged] The coalescence of a binary black hole system is one of the main
sources of gravitational waves that present and future detectors will study.
Apart from the energy and angular momentum that these waves carry, for
unequal-mass binaries there is also a net flux of linear momentum that implies
a recoil velocity of the resulting final black hole in the opposite direction.
We present a computation of the recoil velocity based on the close-limit
approximation scheme, which gives excellent results for head-on and grazing
collisions of black holes when compared to full numerical relativistic
calculations. We obtain a maximum recoil velocity of ~ 64 km/s for a symmetric
mass ratio eta = M_1 M_2/(M_1+M_2)^2 ~ 0.19 and an initial proper separation of
4 M, where M is the total ADM mass of the system. This separation is the
maximum at which the close-limit approximation is expected to provide accurate
results. If we supplement this estimate with PN calculations up to the
innermost stable circular orbit, we obtain a lower bound for the recoil
velocity, with a maximum around 84 km/s. This is a lower bound because it
neglects the initial merger phase. We can however obtain a rough estimate by
using PN methods or the close-limit approximation. Since both methods are known
to overestimate the amount of radiation, we obtain in this way an upper bound
for the recoil with maxima in the range of 220-265 km/s. We also provide
non-linear fits to these estimated upper and lower bounds. These estimates are
subject to uncertainties related to issues such as the choice of initial data
and higher effects in perturbation theory. Nonetheless, our estimates are
consistent with previous results in the literature and suggest a narrower range
of possible recoil velocities.
