Superradiant scattering of electromagnetic waves emitted from disk around Kerr black holes

TL;DR

This paper investigates electromagnetic wave emission from a disk around a Kerr black hole, demonstrating superradiant scattering where energy is extracted from the black hole and transported back to the disk, affecting the observed energy spectrum.

Contribution

It introduces a new method to analyze electromagnetic superradiance from disk-emitted waves around Kerr black holes, including a formula for energy flux and insights into energy transport.

Findings

Energy extracted from the black hole is mainly transported back to the disk.

The energy spectrum of emitted electromagnetic waves is affected by superradiance.

The study provides a framework for understanding electromagnetic superradiance in black hole-disk systems.

Abstract

We study electromagnetic perturbations around a Kerr black hole surrounded by a thin disk on the equatorial plane. Our main purpose is to reveal the black hole superradiance of electromagnetic waves emitted from the disk surface. The outgoing Kerr-Schild field is used to describe the disk emission, and the superradiant scattering is represented by a vacuum wave field which is added to satisfy the ingoing condition on the horizon. The formula to calculate the energy flux on the disk surface is presented, and the energy transport in the disk-black hole system is investigated. Within the low-frequency approximation we find that the energy extracted from the rotating black hole is mainly transported back to the disk, and the energy spectrum of electromagnetic waves observed at infinity is also discussed.