Gravitational lensing by compact objects within plasma

TL;DR

This paper investigates how plasma affects gravitational lensing, revealing frequency-dependent effects that can significantly alter light trajectories near massive objects, especially at low frequencies.

Contribution

It introduces a model for frequency-dependent gravitational lensing in plasma, highlighting the complex behavior of light in inhomogeneous plasma environments near compact objects.

Findings

Plasma causes frequency-dependent shifts in gravitational lensing effects.

Low-frequency rays can be reflected by plasma, acting like a mirror.

Inhomogeneous plasma introduces complex light trajectories even in symmetric cases.

Abstract

Frequency-dependent gravitational lens effects are found for trajectories of electromagnetic rays passing through a distribution of plasma near a massive object. Ray propagation through plasma adds extra terms to the equations of motion that depend on the plasma refractive index. For low-frequency rays these refractive effects can dominate, turning the gravitational lens into a mirror. While light rays behave like particles with an effective mass given by the plasma frequency in a medium with constant density, an inhomogeneous plasma introduces more complicated behavior even for the spherically symmetric case. As a physical example, the pulse profile of a compact object sheathed in a dense plasma is examined, which introduces dramatic frequency-dependent shifts from the behavior in vacuum.