New results for electromagnetic quasinormal and quasibound modes of Kerr black holes

TL;DR

This paper numerically analyzes electromagnetic perturbation spectra of Kerr black holes, identifying quasinormal, quasibound, and spurious modes, enhancing understanding of black hole ring-down signals in astrophysics.

Contribution

It introduces a novel numerical method for solving the spectral system of Kerr perturbations, clearly distinguishing between physical and spurious electromagnetic modes.

Findings

Identified three types of electromagnetic spectra: QNM, QBM, and spurious modes.

Developed a direct boundary condition imposition method for spectral analysis.

Clarified the nature of spurious spectra in Kerr black hole perturbations.

Abstract

The perturbations of the Kerr metric and the miracle of their exact solutions play a critical role in the comparison of predictions of general relativity with astrophysical observations of compact massive objects. The differential equations governing the late-time ring-down of the perturbations of the Kerr metric, the Teukolsky Angular Equation and the Teukolsky Radial Equation, can be solved analytically in terms of confluent Heun functions. In this article, we solve numerically the spectral system formed by those exact solutions and we obtain the electromagnetic (EM) spectra of the Kerr black hole. Because of the novel direct way of imposing the boundary conditions, one is able to discern three different types of spectra: the well-known quasinormal modes (QNM), the symmetric with respect to the real axis quasibound modes (QBM) and a spurious spectrum who is radially unstable. This approach allows clearer justification of the term "spurious" spectrum, which may be important considering the recent interest in the spectra of the electromagnetic counterparts of events producing gravitational waves.