Shadow of axisymmetric, stationary and asymptotically flat black holes in the presence of plasma

TL;DR

This paper investigates how plasma surrounding rotating black holes affects their shadow, deriving a general shape formula and analyzing specific geometries, revealing plasma causes smaller, less deformed shadows.

Contribution

It provides a general expression for black hole shadow shapes in plasma environments and applies it to various geometries, including Kerr-Newman-like and scalar-tensor Einstein-Gauss-Bonnet black holes.

Findings

Plasma reduces the size of black hole shadows.

Plasma makes the shadows less deformed.

The shape formula applies to separable Hamilton-Jacobi equations.

Abstract

We study the shadow produced by a class of rotating black holes surrounded by plasma. The metric for these black holes arises by applying the Newman-Janis algorithm to a family of spherically symmetric spacetimes, which includes several well known geometries as special cases. We derive a general expression for the shape of the shadow in the case that the plasma frequency leads to a separable Hamilton-Jacobi equation for light. We present two examples in which we obtain the shadow contours and the observables resulting from them. In one, we analyze Kerr-Newman-like geometries, including braneworld and Horndeski gravity black holes, while in the other, we consider scalar-tensor 4D Einstein-Gauss-Bonnet gravity spacetimes. In both cases, we find that the presence of plasma leads to a smaller and less deformed shadow.