### 5.1 Studies of black hole QNM excitation

In the study of QNMs of black holes the attention, in most cases, was focused on estimating the
spectrum and its properties. But there is limited work in the direction of understanding what details of the
perturbation determine the strength of the QNM ringing.
In 1977 Detweiler [76] discussed the resonant oscillations of a rotating black hole, and after identifying
the QNMs as “resonance peaks” in the emitted spectrum he showed that the modes formally correspond to
poles of a Green function to the inhomogeneous Teukolsky equation [197]. This idea has been extended in a
more mathematically rigorous way by Leaver [136]. Leaver extracts the QNM contribution to the emitted
radiation as a sum over residues. This sum arises when the inversion contour of the Laplace transform,
which was used to separate the dependence on the spatial variables from the time dependence, is deformed
analytically in the complex frequency plane. In this way the contribution from the QNM can be
accounted for. Sun and Price [194, 195] discussed in detail the way that QNM are excited by
given Cauchy data based to some extent on numerical results obtained by Leaver [136]. Lately,
Andersson [12] used the phase-integral method to determine some characteristics of the QNM
excitation.