Abstract
When emitted from distant galaxies, light rays may propagate through regions with matter (especially plasma), not only through vacuum. A precise specification of the medium is important - it can cause the rays to change substantially their form.
If the rays are passing near a strongly gravitating compact object, both plasma and gravitational effects have to be taken into account. The impact of refractive and dispersive media around compact objects is typically relevant in problems related to the gravitational lensing.
In our work we focus on how the regions allowed for propagation of rays in the equatorial plane of a rotating black hole are modified due to the presence of the refractive and dispersive media with various density profiles. We investigate the media with the density distributions corresponding to typical profiles of lensing galaxies, as well as with the density distribution of a nonsingular isothermal sphere.
We show that the regions permitted for the rays in a medium are substantially reduced as compared with the corresponding regions in vacuum. We also study allowed regions as seen by observers freely falling onto the black hole from the rest at infinity.