Publication at Faculty of Mathematics and Physics |

2022

Bending of light rays propagating in a strong gravitational field is one of the crucial predictions of general relativity. This phenomenon has been currently widely studied mainly in terms of gravitational lensing.

However, due to increasingly precise measurements, to be able to compare theory with observation, effects on the light ray trajectories caused by the presence of a medium, typically of refractive and dispersive properties, surrounding a gravitating object, have to be considered. In our study, both gravitational and plasma effects on the light rays are investigated in terms of the deflection angle which can significantly differ from the vacuum case.

A general formula for the light deflection angle in an axially symmetric spacetime in plasma is given. Its application is further demonstrated for particular cases, especially for the Kerr and Hartle-Thorne metrics.

Effects of plasma on the light propagation can thus be quantified. An approximate analytical formula for the deflection angle in a weak gravitational field is derived for these spacetimes with plasma, and it is compared with some exact solutions.

Moreover, light trajectories for several initial conditions are obtained and visualised explicitly. The inaccuracy stemming from an approximate solution is discussed.