Distorted black holes radiate gravitational waves. In the so-called ringdown
phase radiation is emitted in a discrete set of complex quasinormal
frequencies, whose values depend only on the black hole's mass and angular
momentum. Ringdown radiation could be detectable with large signal-to-noise
ratio by the Laser Interferometer Space Antenna LISA. If more than one mode is
detected, tests of the black hole nature of the source become possible. The
detectability of different modes depends on their relative excitation, which in
turn depends on the cause of the perturbation (i.e. on the initial data). A
``universal'', initial data-independent measure of the relative mode excitation
is encoded in the poles of the Green's function that propagates small
perturbations of the geometry (``excitation factors''). We compute for the
first time the excitation factors for general-spin perturbations of Kerr black
holes. We find that for corotating modes with l=m the excitation factors tend
to zero in the extremal limit, and that the contribution of the overtones
should be more significant when the black hole is fast rotating. We also
present the first analytical calculation of the large-damping asymptotics of
the excitation factors for static black holes, including the Schwarzschild and
Reissner-Nordstrom metrics. This is an important step to determine the
convergence properties of the quasinormal mode expansion.Comment: 33 pages, 9 figures, 7 tables, RevTeX4. v2: Two new figures and minor
changes in the presentation. Matches version in press in Phys. Rev.