  Integrating trajectories of low-mass X-ray binaries containing black holes
within the Galactic potential, Repetto, Davies & Sigurdsson recently showed
that the large distances of some systems above the Galactic plane can only be
explained if black holes receive appreciable natal kicks. Surprisingly, they
found that the distribution of black hole kick velocities (rather than that of
the momenta) should be similar to that of neutron stars. Here I argue that this
result can be understood if neutron star and black hole kicks are a consequence
of large-scale asymmetries created in the supernova ejecta by the explosion
mechanism. The corresponding anisotropic gravitational attraction of the
asymmetrically expelled matter does not only accelerate new-born neutron stars
by the "gravitational tug-boat mechanism". It can also lead to delayed
black-hole formation by asymmetric fallback of the slowest parts of the initial
ejecta onto the transiently existing neutron star, in course of which the
momentum of the black hole can grow with the fallback mass. Black hole kick
velocities will therefore not be reduced by the ratio of neutron star to black
hole mass as would be expected for kicks caused by anisotropic neutrino
emission of the nascent neutron star.
