Gravitational perturbations of the Kerr geometry: High-accuracy study

TL;DR

This paper introduces a high-precision computational method for analyzing gravitational perturbations of Kerr black holes, focusing on quasinormal modes near extremal Kerr and special frequencies, with results aligning well with analytic predictions.

Contribution

A novel high-accuracy spectral method for solving the angular Teukolsky equation to study Kerr quasinormal modes, including near extremal and Schwarzschild limit behaviors.

Findings

Good agreement with analytic predictions near accumulation points in extremal Kerr.

Successful approach to the Schwarzschild limit with high precision.

Insights into the existence of quasinormal modes at special frequencies using confluent Heun polynomials.

Abstract

We present results from a new code for computing gravitational perturbations of the Kerr geometry. This new code carefully maintains high precision to allow us to obtain high-accuracy solutions for the gravitational quasinormal modes of the Kerr space-time. Part of this new code is an implementation of a spectral method for solving the angular Teukolsky equation that, to our knowledge, has not been used before for determining quasinormal modes. We focus our attention on two main areas. First, we explore the behavior of these quasinormal modes in the extreme limit of Kerr, where the frequency of certain modes approaches accumulation points on the real axis. We compare our results with recent analytic predictions of the behavior of these modes near the accumulation points and find good agreement. Second, we explore the behavior of solutions of modes that approach the special frequency $M\omega=-2i$ in the Schwarzschild limit. Our high-accuracy methods allow us to more closely approach the Schwarzschild limit than was possible with previous numerical studies. Unlike previous work, we find excellent agreement with analytic predictions of the behavior near this special frequency. We include a detailed description of our methods, and make use of the theory of confluent Heun differential equations throughout. In particular, we make use of confluent Heun polynomials to help shed some light on the controversy of the existence, or not, of quasinormal and total-transmission modes at certain special frequencies in the Schwarzschild limit.