Absorption of planar massless scalar waves by Kerr black holes

TL;DR

This paper analyzes how Kerr black holes absorb planar massless scalar waves at various angles, revealing frequency-dependent behaviors and the influence of black hole rotation and wave alignment.

Contribution

It extends the partial wave and complex angular momentum methods to analyze scalar wave absorption by Kerr black holes at arbitrary angles, including new approximations and symmetry considerations.

Findings

Low-frequency cross section approaches horizon area.

High-frequency cross section approaches geodesic capture cross section.

Counterrotating waves experience greater absorption.

Abstract

We consider planar massless scalar waves impinging upon a Kerr black hole, for general angles of incidence. We compute the absorption cross section via the partial wave approach, and present a gallery of results. In the low-frequency regime, we show that the cross section approaches the horizon area; in the high-frequency regime, we show that the cross section approaches the geodesic capture cross section. In the aligned case, we extend the complex angular momentum method to obtain a `sinc' approximation, which relates the regular high-frequency oscillations in the cross section to the properties of the polar null orbit. In the non-aligned case, we show, via a semi-analytic approximation, that the reduction in symmetry generates a richer, less regular absorption cross section. We separate the absorption cross section into corotating and counterrotating contributions, showing that the absorption is larger for counterrotating waves, as expected.