ABSTRACT
Current efforts of the Event Horizon Telescope aim at resolving the black hole shadow, mainly with the goal to probe the nature of the black hole and thus the underlying theory of gravity. Here, including other lensing features, in particular multiple images either of individual sources or in form of the so-called photon rings, will allow to place much tighter constrains on the spacetime parameters, in particular the spin. In my talk I will use the Kerr spacetime as an example to show how we can use low and very high-order images to constrain the spin of a black hole. For this purpose I will first solve the equations of motion for lightlike geodesics analytically using elementary and Jacobi's elliptic functions as well as Legendre's elliptic integrals. Then I will place a standard observer in the domain of outer communication and relate the constants of motion of the lightlike geodesics to latitude-longitude coordinates on the observer's celestial sphere. I will use the analytical solutions to write down a lens equation, and to calculate the travel time and the redshift for three different types of light sources. I show that in the case of near-extremal spins close to the boundary of the shadow corotating light rays lead to the formation of very thin but still well-defined layers of very high-order images and discuss how combined effects of these images may be observable. While in the main part of my talk I will focus on gravitational lensing of light rays I will also discuss potential applications to gravitational lensing of high-frequency gravitational waves.
BIO
Torben C. Frost received his Ph.D. in 2022 from the University of Bremen. After a short-term postdoc position at ZARM, University of Bremen, he joined the KIAA at Peking University as postdoctoral researcher in March 2023.
Host: Pablo Renard
