Extremal rotating black holes, scalar perturbation and superradiant stability

TL;DR

This paper investigates the conditions under which extremal Kerr and Kerr-Newman black holes remain stable against superradiant scalar perturbations, identifying parameter regimes that prevent instability.

Contribution

It provides a detailed analysis of superradiant stability regions for extremal Kerr and Kerr-Newman black holes under massive and charged scalar fields, extending previous stability criteria.

Findings

Kerr black holes are superradiantly stable if <//

Kerr-Newman black holes are stable when <qQ/M and  > rac{\u221a{3k^2+2}}{\u221a{k^2+2}}, with k=a/M.

Stability conditions depend on scalar field frequency, mass, charge, and black hole parameters.

Abstract

A (charged) rotating black hole may be unstable against a (charged) massive scalar field perturbation due to the existence of superradiance modes. The stability property depends on the parameters of the system. In this paper, the superradiant stable parameter space is studied for the four-dimensional extremal Kerr and Kerr-Newman black holes under massive and charged massive scalar perturbation. For the extremal Kerr case, it is found that when the angular frequency and proper mass of the scalar perturbation satisfy the inequality $\omega<\mu/\sqrt{3}$, the extremal Kerr black hole and scalar perturbation system is superradiantly stable. For the Kerr-Newman black hole case, when the angular frequency of the scalar perturbation satisfies $\omega<qQ/M$ and the product of the mass-to-charge ratios of the black hole and scalar perturbation satisfies $\frac{\mu}{q}\frac{M}{Q} > \frac{\sqrt{3 k^2+2} }{ \sqrt{k^2+2} },~k=\frac{a}{M}$, the extremal Kerr-Newman black hole is superradiantly stable under charged massive scalar perturbation.