Absorption by dirty black holes: null geodesics and scalar waves

TL;DR

This paper investigates how matter surrounding a black hole affects the absorption and scattering of scalar waves, revealing that the absorption cross section varies with frequency and depends on the black hole's environment.

Contribution

It provides the first analysis of scalar wave absorption by a black hole with surrounding matter, extending understanding beyond isolated black hole models.

Findings

Low-frequency absorption cross section equals horizon area.

High-frequency absorption approaches geodesic capture cross section.

Surrounding matter influences wave absorption spectra.

Abstract

Black holes are a paradigm in nowadays physics, and are expected to be hosted at the center of galaxies. Supermassive galactic black holes are not isolated, and their surroundings play crucial roles in many observational features. The absorption and scattering of fields by isolated black holes have been vastly studied, allowing the understanding of many phenomenological features. However, as far as we are aware, a study of the influence of the presence of matter surrounding black holes in its planar wave scattering and absorption spectrum is still lacking in the literature. This may be important in the analysis of, for instance, the accretion of dark matter by black holes. We consider planar massless scalar waves incident upon a Schwarzschild black hole surrounded by a thin spherical shell. We use the partial wave method to determine the absorption cross section, and present a selection of numerical results. In the low-frequency regime, we show that the absorption cross section is equal to the horizon area. At the high-frequency regime, we show that the absorption cross section approaches the geodesic capture cross section.