Numerical stability of the electromagnetic quasinormal and quasibound modes of Kerr black holes

TL;DR

This paper investigates the numerical stability of electromagnetic quasinormal and quasibound modes of Kerr black holes, emphasizing the importance of the confluent Heun functions in distinguishing physical spectra from artifacts.

Contribution

It introduces an epsilon-method analysis of the spectra stability for QNM and QBM in Kerr black holes using confluent Heun functions, identifying physical and unphysical modes.

Findings

QNM and QBM are stable in certain complex plane regions

The third spectrum is numerically unstable and unphysical

Understanding the Heun functions framework is crucial for accurate spectral analysis

Abstract

The proper understanding of the electromagnetic counterpart of gravity-waves emitters is of serious interest to the multimessenger astronomy. In this article, we study the numerical stability of the quasinormal modes (QNM) and quasibound modes (QBM) obtained as solutions of the Teukolsky Angular Equation and the Teukolsky Radial Equation with appropriate boundary conditions. We use the epsilon-method for the system featuring the confluent Heun functions to study the stability of the spectra with respect to changes in the radial variable. We find that the QNM and QBM are stable in certain regions of the complex plane, just as expected, while the third "spurious" spectrum was found to be numerically unstable and thus unphysical. This analysis shows the importance of understanding the numerical results in the framework of the theory of the confluent Heun functions, in order to be able to distinguish the physical spectra from the numerical artifacts.