Revisiting the shadow of a black hole in the presence of a plasma

TL;DR

This paper investigates how a plasma surrounding a black hole influences its shadow, revealing size and shape changes, frequency-dependent effects, and the emergence of multi-ring structures.

Contribution

It introduces a renewed ray-tracing algorithm to analyze plasma effects on black hole shadows, highlighting new frequency-dependent phenomena and complex shadow structures.

Findings

Plasma alters the size and shape of the black hole shadow.

High-frequency photons are minimally affected by plasma.

Low-frequency photons experience significant trajectory deviations and multi-ring shadow structures.

Abstract

We study the photon's motion around a black hole in the presence of a plasma whose density is a function of the radius coordinate by a renewed ray-tracing algorithm and investigate the influence of the plasma on the shadow of the black hole. The presence of plasma affects not only the size but also the shape of black hole shadow. Furthermore, the influence of plasma on trajectories of photons depends on the frequency of photon. For the high-frequency photons, the influence is negligible, on the contrary, the trajectories of low-frequency photons is affected significantly by the plasma. Interestingly, it is also found that the black hole image would take on a multi-ring structure due to the presence of plasma.