Light rings on stationary axisymmetric spacetimes: blind to the topology and able to coexist

Abstract

It has been established that Black Hole (BH) spacetimes obeying some general set of assumptions always possess, at least, one light ring (per rotation sense) [arXiv:2003.06445]. This theorem was originally established for asymptotically flat, stationary, axial symmetric, 1+3 dimensional circular spacetimes harbouring a non-extremal and topologically spherical Killing horizon. Following the mantra that a theorem is only as strong as its assumptions in this work we extend this theorem to non topologically spherical (toroidal) BHs and to spacetimes harbouring more than one BH. As in [arXiv:2003.06445], we show that each BH still contributes with, at least, one LR (per rotation sense).