Application of the confluent Heun functions for finding the quasinormal modes of nonrotating black holes

Although finding numerically the quasinormal modes of a nonrotating black hole is a well-studied question, the physics of the problem is often hidden behind complicated numerical procedures aimed at avoiding the direct solution of the spectral system in this case. In this article, we use the exact analytical solutions of the Regge-Wheeler equation and the Teukolsky radial equation, written in terms of confluent Heun functions. In both cases, we obtain the quasinormal modes numerically from spectral condition written in terms of the Heun functions. The frequencies are compared with ones already published by Andersson and other authors. A new method of studying the branch cuts in the solutions is presented -- the epsilon-method. In particular, we prove that the mode $n=8$ is not algebraically special and find its value with more than 6 firm figures of precision for the first time. The stability of that mode is explored using the $ε$ method, and the results show that this new method provides a natural way of studying the behavior of the modes around the branch cut points.

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