Photon regions in stationary axisymmetric spacetimes and umbilic conditions

Abstract

We present the fundamentals of the recently proposed geometric description of photon regions in terms of foliation into fundamental photon hypersurfaces, which satisfies the umbilic condition for the subbundle of the tangent bundle defined by the generalized impact parameter.

14 pages, to be submitted to the Proceedings of the 16th Marcel Grossmann meeting