Quasinormal modes in Kerr spacetime as a 2D Eigenvalue problem

Abstract

We revisit the computation of quasinormal modes (QNMs) of the Kerr black hole using a numerical approach exploiting a representation of the Teukolsky equation as a $2D$ elliptic partial differential equation. By combining the hyperboloidal framework with a $m$-mode decomposition, we recast the QNM problem into a genuine eigenvalue problem for each azimuthal mode. This formulation enables the simultaneous extraction of multiple QNMs, traditionally labelled by overtone number $n$ and angular index $\ell$, without requiring prior assumptions about their structure. We advocate for a simplified notation in which each overtone is uniquely labelled by a single index $q$, thereby avoiding the conventional but artificial distinction between regular and mirror modes. We compare two distinct hyperboloidal gauges-radial fixing and Cauchy horizon fixing-and demonstrate that, despite their different geometric properties and behaviour in the extremal limit, they yield numerical values for the QNM spectra with comparable accuracy and exponential convergence. Moreover, we show that strong gradients observed near the horizon in the extremal Kerr regime are coordinate artefacts of specific slicing rather than physical features. Finally, we investigate the angular structure of the QNM eigenfunctions and show that the $m$-mode approach allows flexible projection onto both spin-weighted spheroidal and spherical harmonic bases. These results underscore the robustness and versatility of the hyperboloidal $m$-mode method as a foundation for future studies of QNM stability, pseudospectra, and mode excitation in gravitational wave astronomy.