Superradiant instability for black holes immersed in a magnetic field

TL;DR

This paper investigates how a magnetic field influences the superradiant instability of massive scalar fields around rotating black holes, showing that the magnetic field can significantly amplify the instability growth rate.

Contribution

It demonstrates that magnetic fields add an effective mass term to the scalar potential, enhancing the superradiant instability in Kerr black holes with slow rotation.

Findings

Magnetic field amplifies the instability growth rate.

Effective mass term B|m| influences scalar field behavior.

Analysis of quasinormal modes in magnetized black holes.

Abstract

We consider the bound states of the massive scalar field around a rotating black hole immersed in the asymptotically uniform magnetic field. In the regime of slow black hole rotation, the Klein-Gordon equation allows separation of variables. We show that the growth rate of the instability can be amplified a few times by the magnetic field. The effect occurs because the magnetic field adds the "effective mass" term $B |m|$ to the scalar field potential for a Kerr black hole. In addition, and as a by-product, we discuss the behavior of the quasinormal modes for the magnetized rotating black holes.