Gravitational Lensing in the Schwarzschild Spacetime: Photon Rings in Vacuum and in the Presence of a Plasma

Astrophysical black hole candidates are often surrounded by an accretion disk. In particular the interior region of this accretion disk can consist of a plasma and the trajectories of light rays travelling through this plasma can deviate significantly from the trajectories of light rays travelling through vacuum. While usually these environments are very complex we can already learn a lot about the observable features using simple plasma models. In the context of general relativity for some of these plasma models the equations of motion are fully separable and even analytically solvable. In my talk I will illustrate what we can learn from such models using the Schwarzschild spacetime as background. I will assume that the black hole is surrounded by an inhomogeneous, pressureless, and non-magnetised plasma and solve the equations of motion analytically exact. Then I will assume that we have a luminous disk in the equatorial plane and discuss the impact of the plasma on the so-called photon rings. I will discuss the changes of their geometrical structure, the redshift, and the travel time of the photons compared to photon rings in vacuum and what we can learn about the properties of the plasma. I will also discuss how the presence of the plasma may be able to help us to constrain gravity in the strong field regime. Finally, I will discuss how the obtained results will contribute to designing a multimessenger approach for probing gravity in the strong field regime in the context of the Maxwell-Einstein-Pauli Observatory.