Extremal Charged Black Holes and Superradiantly Unstable Quasinormal Modes

TL;DR

This paper demonstrates that extremal charged black holes can exhibit superradiant instability through unstable quasinormal modes, especially when the effective potential resembles a volcano shape, revealing new instability mechanisms.

Contribution

It numerically shows that extremal charged black holes can excite higher-frequency superradiant unstable modes, linking instability to the shape of the effective potential.

Findings

Unstable modes are associated with volcano-shaped potentials.

Higher-frequency superradiant instabilities can occur in certain charge configurations.

The instability mechanism is related to the potential's shape in the wave equation.

Abstract

It was recently shown that the extremal charged black holes in STU supergravity suffer from superradiant instability owing to the existence of the unstable (low-frequency) quasibound states associated with a charged massive scalar field. In this paper, we show numerically that for some charge configurations, these black holes can also excite the (higher-frequency) superradiantly unstable quasinormal modes. We find empirically that the unstable modes are closely related to having a volcano-shaped effective potential in the Schr\"odinger-like wave equation.