Topological charge and black hole photon spheres in massive gravity
In this paper, we investigate the existence and nature of the photon spheres (PS) in the background of four dimensional static and spherically symmetric black holes in the de Rham-Gabadadze-Tolley (dRGT) massive gravity theory. Apart from the known case of one PS, there are regions in the parameter space of massive gravity where either two or no PS's exist outside the event horizon. Topological arguments show that, the case of one PS falls in the category of Einstein gravity (with topological charge $-1$), whereas, the cases with two or zero PS's belong to a different topological class with total charge zero. PS's of horizonless compact objects, also belong to the same class with total topological charge $0$, though, one distinction can be made with the black holes in massive gravity. While in the former case, the inner PS is stable, in the later case, it is the outer PS which is stable (the inner PS is unstable). We also study the landscape of possible regions of existence of standard and exotic photon spheres in the massive gravity parameter space, and correlate it with the horizon structure of the black holes.