Absorption of electromagnetic plane waves by rotating black holes

TL;DR

This paper investigates how rotating black holes absorb electromagnetic waves from various directions, revealing polarization-dependent absorption and conditions for superradiant emission, with numerical results matching theoretical approximations.

Contribution

It provides detailed numerical analysis of electromagnetic wave absorption by Kerr black holes, including polarization effects and superradiance conditions, extending previous low- and high-frequency theories.

Findings

Circular polarization absorption depends on black hole spin.

Superradiant emission can surpass absorption under specific conditions.

Absorption varies with wave frequency and incidence angle.

Abstract

We study the absorption of monochromatic electromagnetic plane waves impinging upon a Kerr black hole, in the general case that the direction of incidence is not aligned with the black hole spin axis. We present numerical results that are in accord with low- and high-frequency approximations. We find that circularly-polarized waves are distinguished by the black hole spin, with counter-rotating polarizations being more absorbed than co-rotating polarizations. At low frequencies and moderate incidence angles, there exists a narrow parameter window in which superradiant emission in the dipole mode can exceed absorption in the non-superradiant modes, allowing a planar electromagnetic wave to stimulate net emission from a black hole.