Exceptional lines in the Kerr-Newman black hole spectrum

Abstract

We investigate massive scalar perturbations of Kerr-Newman black holes, focusing on the $(\ell,m) = (1,1)$ quasinormal mode spectrum in the near-extremal regime. We identify a sequence of exceptional lines, at which overtone frequencies become degenerate, together with a corresponding sequence of exceptional points for massless fields. We analyze the geometric phases associated with these exceptional points by transporting the spectrum around closed loops in parameter space and examining the resulting permutation of quasinormal mode frequencies. We further show that these degeneracies are closely related to the branching of the spectrum into zero-damping and damped modes, and derive an analytic expression for the frequencies of the zero-damping modes in the extremal limit.