Speaker
Description
Analytical calculations of gravitational lensing by black holes and other compact or ultracompact objects, in particular of the shadow, were originally always done under the assumption that light rays are lightlike geodesics of the spacetime metric, i.e., that effects of a medium can be neglected. More recently the effect of a plasma was taken into account in numerous papers, where the plasma was always considered as being 'cold', i.e, its absolute temperature was treated as zero and its pressure was neglected. In this talk I review these earlier results and I generalise them to the case of a warm plasma, where the temperature is assumed to be non-zero but small enough such that the relevant equations can be linearised with respect to it. The temperature effects turn out to be small but possibly non-negligible in some cases of astrophysical interest. - This talk is largely based on joint work with Barbora Bezdekova, Haifa University, Israel.