Quasinormal-mode encounters: Pseudospectra near Kerr--Newman exceptional points

Abstract

Exceptional points (EPs), at which two quasinormal frequencies and their eigenvectors coalesce, can enhance the spectral instability of black hole quasinormal modes. We investigate this behavior in Kerr--Newman spacetimes through pseudospectra and condition numbers for neutral massless scalar fields and coupled gravitoelectromagnetic (GE) perturbations. For the scalar field, we follow the $(\ell,m)=(1,1)$ spheroidal branch and derive the energy norm from the stress-energy tensor. For GE perturbations, we use a coupled four-field system equipped with a positive radiative energy. Along matched parameter paths toward the selected EPs, the frequency splitting follows a square-root law and the condition numbers grow as the inverse square root of the distance to the EP. Their product, which approximates the norm of the double-pole coefficient, does not depend on the offset or, for GE, on the angular truncation and projection used. The pseudospectral contours resolve the transition from individual-mode sensitivity to the collective response of the pair. At the selected backgrounds and resolution, both pair members are more sensitive than two more-damped modes at the same background, but this ordering holds only below a crossover offset that decreases with radial resolution. For the GE pair, angular refinement also displaces the coalescence by more than the selected offset. Within the discretized problems and energy norms considered here, these results describe qualitative trends of spectral instability near EPs rather than continuum magnitudes, and motivate further study of their possible role in black-hole ringdown.