N. Andersson et H. Onozawa, QUASI-NORMAL MODES OF NEARLY EXTREME REISSNER-NORDSTROM BLACK-HOLES, Physical review. D. Particles and fields, 54(12), 1996, pp. 7470-7475
We present detailed calculations of the quasinormal modes of Reissner-
Nordstrom black holes. While the first few, slowly damped, modes depen
d on the charge of the black hole in a relatively simple way, we find
that the rapidly damped modes show several peculiar features. The high
er modes generally spiral into the value for the extreme black hole as
the charge increases. We also discuss the possible existence of a pur
ely imaginary mode for the Schwarzschild black hole: Our data suggest
that there is a quadrupole quasinormal mode that limits to omega M=-2i
as Q-->0.