Excitation factors and exceptional points of Kerr--Newman quasinormal modes

Abstract

We study excitation factors and investigate exceptional points (EPs) of Kerr--Newman quasinormal modes using the exact separated gravitoelectromagnetic equations. At fixed charge, the magnitudes of the excitation factors for the gravitational-led modes studied here decrease with increasing spin, whereas those for electromagnetic-led modes exhibit both monotonic and nonmonotonic behavior. The magnitude of the excitation factor for the first overtone can be comparable to or larger than that for the fundamental mode. We identify 42 EPs at which pairs of modes coalesce in the gravitational-led and electromagnetic-led sectors. Some EPs form sequences involving successive overtone pairs, with higher-overtone pairs coalescing progressively closer to the extremal limit. The additional EPs identified outside these sequences all share the same mode-pair labels as sequence members, but occur at different values of the black hole spin and charge. These findings clarify how rotation and charge shape the relative excitation strengths of different modes and the pattern of mode coalescence in the non-Hermitian spectrum of Kerr--Newman black holes.