Influence of a plasma on the shadow of a spherically symmetric black hole

TL;DR

This paper analytically explores how a non-magnetized, pressure-less plasma affects the observable shadow of spherically symmetric, static black holes and wormholes, providing formulas applicable to various spacetime models.

Contribution

It derives general formulas for plasma influence on black hole shadows in spherically symmetric, static spacetimes, including Schwarzschild black holes and Ellis wormholes, considering specific plasma distributions.

Findings

Plasma can significantly alter the size and shape of black hole shadows.

The formalism applies to both black holes and wormholes, broadening observational implications.

Radial free-fall plasma models show distinct shadow modifications.

Abstract

We analytically calculate the influence of a plasma on the shadow of a black hole (or of another compact object). We restrict to spherically symmetric and static situations, where the shadow is circular. The plasma is assumed to be non-magnetized and pressure-less. We derive the general formulas for a spherically symmetric plasma density on an unspecified spherically symmetric and static spacetime. The formalism applies not only to black holes but also, e.g., to wormholes. As examples for the underlying spacetime model, we consider the Schwarzschild spacetime and the Ellis wormhole. In particular, we treat the case that the plasma is in radial free fall from infinity onto a Schwarzschild black hole. The perspectives of actually observing the influence of a plasma on the shadows of supermassive black holes are discussed.