Eigenvalue repulsions and quasinormal mode spectra of Kerr-Newman: an extended study

TL;DR

This paper provides an extended analytical and numerical study of quasinormal modes of Kerr-Newman black holes, focusing on eigenvalue repulsion, near-horizon approximations, and the full perturbation system, enhancing understanding of black hole perturbation spectra.

Contribution

It offers a comprehensive derivation of the perturbation equations, compares frequency approximations with numerical data, and elucidates the eigenvalue repulsion phenomenon in Kerr-Newman black holes.

Findings

Analytical formula for QNM frequencies near extremality

Confirmation of eigenvalue repulsion in QNM spectra

More damped QNM families identified

Abstract

The frequency spectra of the gravito-electromagnetic perturbations of the Kerr-Newman (KN) black hole with the slowest decay rate have been computed recently. It has been found that KN has two families $-$ the photon sphere and the near-horizon families $-$ of quasinormal modes (QNMs), which display the interesting phenomenon of eigenvalue repulsion. The perturbation equations, in spite of being a coupled system of two PDEs, are amenable to an analytic solution using the method of separation of variables in a near-horizon expansion around the extremal KN black hole. This leads to an analytical formula for the QNM frequencies that provides an excellent approximation to the numerical data near-extremality. In the present manuscript we provide an extended study of these properties that were not detailed in the original studies. This includes: 1) a full derivation of a gauge invariant system of two coupled PDEs that describes the perturbation equations \cite{Dias:2015wqa}, 2) a derivation of the eikonal frequency approximation \cite{Zimmerman:2015trm,Dias:2021yju} and its comparison with the numerical QNM data, 3) a derivation of the near-horizon frequency approximation \cite{Dias:2021yju} and its comparison with the numerical QNMs, and 4) more details on the phenomenon of eigenvalue repulsion (also known as \emph{level repulsion}, \emph{avoided crossing} or \emph{Wigner-Teller effect}) and a first principles understanding of it that was missing in the previous studies. Moreover, we provide the frequency spectra of other KN QNM families of interest to demonstrate that they are more damped than the ones we discuss in full detail.