Quasinormal modes of Kerr-Newman black holes: revisiting the Dudley-Finley approximation

The Kerr-Newman spacetime is the unique vacuum solution of the Einstein–Maxwell theory in four spacetime dimensions representing a stationary, axisymmetric and asymptotically-flat black hole. For this reason, studying its stability properties is important. In this presentation we report on a comprehensive study of the quasinormal mode spectrum of Kerr-Newman black holes in the Dudley-Finley approximation, where the linear gravitoelectromagnetic perturbations are decoupled by "freezing" either one of the fields to its background value. We reassess the accuracy of this approximation and study the spectral features in the near-extremal limit. By comparing our results to recent calculations that solve the full system of coupled perturbations, we show that the Dudley-Finley approximation captures the main features of the spectrum.