Gravitational lensing in plasma: Relativistic images at homogeneous plasma

TL;DR

This paper studies how a uniform plasma around a Schwarzschild black hole affects gravitational lensing, showing that plasma increases the size and brightness of relativistic images, with effects depending on photon frequency.

Contribution

It derives asymptotic formulas for large deflection angles in plasma and analyzes their impact on relativistic images, extending gravitational lensing theory to plasma environments.

Findings

Plasma increases the angular size of relativistic rings.

Plasma enhances the magnification of relativistic images.

Effects are significant near the plasma frequency.

Abstract

We investigate the influence of plasma presence on relativistic images formed by Schwarzschild black hole lensing. When a gravitating body is surrounded by a plasma, the lensing angle depends on a frequency of the electromagnetic wave due to refraction properties, and the dispersion properties of the light propagation in gravitational field in plasma. The last effect leads to difference, even in uniform plasma, of gravitational deflection angle in plasma from vacuum case. This angle depends on the photon frequency, what resembles the properties of the refractive prism spectrometer. Here we consider the case of a strong deflection angle for the light, traveling near the Schwarzschild black hole, surrounded by a uniform plasma. Asymptotic formulae are obtained for the case of a very large deflection angle, exceeding $2\pi$. We apply these formulae for calculation of position and magnification of relativistic images in a homogeneous plasma, which are formed by the photons performing one or several revolutions around the central object. We conclude that the presence of the uniform plasma increases the angular size of relativistic rings or the angular separation of point images from the gravitating center. The presence of the uniform plasma increases also a magnification of relativistic images. The angular separation and the magnification become significantly larger than in the vacuum case, when the photon frequency goes to a plasma frequency.