Photon spheres in Einstein and Einstein-Gauss-Bonnet theories and circular null geodesics in axially symmetric spacetimes

EMANUEL GALLO; JOSE R VILLANUEVA

PHYSICAL REVIEW D

AMER PHYSICAL SOC

Lugar: New York; Año: 2015 vol. 92 p. 64048 - 64048

1550-7998

In this article we extend a recent theorem proven by Hod (Phys. Lett. B, 727, 345--348, 2013) to n-dimensional Einstein and Einstein-Gauss-Bonnet theories, which gives an upper bound for the photon sphere radii of spherically symmetric black holes. As applications of these results we give a universal upper bound for the real part of quasinormal modes in the WKB limit and a universal lower bound for the positionof the first relativistic image in the strong lensing regime produced by these type of black holes.For the axially symmetric case, we also make some general comments (independent of the underlying gravitational theory) on the relation between circular null geodesics and the fastest way to circle a black hole.