# Effect of black hole--plasma system on light beams

## TL;DR

This paper investigates how a non-magnetized cold plasma around a Kerr black hole affects the propagation of light rays and the observed star distribution near the black hole, using a generalized deviation equation.

## Contribution

It derives a generalized deviation equation for light rays in plasma around Kerr black holes and analyzes the plasma's impact on observed star distributions.

## Key findings

- Plasma modifies the angular distribution of stars near the black hole.
- The deviation of light rays is affected by plasma density and distribution.
- Key features of star distribution are identified and discussed.

## Abstract

In the paper we discuss propagation of light around Kerr black hole surrounded by non-magnetized cold plasma with infinite conductivity. For that purpose, we use equations for propagation of light rays obtained within Synge's approach in the approximation of geometrical optics. We derive equation of deviation of a ray propagating close to the reference ray, which is a generalization of the well-known Jacobi equation, and use it to calculate the modification of angular distribution of stars observed close to a black hole surrounded by plasma, compared to the uniform star distribution that would be seen without black hole or plasma. We place the observer in the equatorial plane of the Kerr black hole and try various choices of plasma distributions described by mathematically simple formulae. Key features of star distribution on a local sky near the black hole are identified and the influence of plasma on them is discussed.