Does the black hole shadow probe the event horizon geometry?

Abstract

There is an exciting prospect of obtaining the shadow of astrophysical black holes (BHs) in the near future with the Event Horizon Telescope. As a matter of principle, this justifies asking how much one can learn about the BH horizon itself from such a measurement. Since the shadow is determined by a set of special photon orbits, rather than horizon properties, it is possible that different horizon geometries yield similar shadows. One may then ask how sensitive is the shadow to details of the horizon geometry? As a case study, we consider the double Schwarzschild BH and analyse the impact on the lensing and shadows of the conical singularity that holds the two BHs in equilibrium -herein taken to be a strut along the symmetry axis in between the two BHs. Whereas the conical singularity induces a discontinuity of the scattering angle of photons, clearly visible in the lensing patterns along the direction of the strut's location, it produces no observable effect on the shadows, whose edges remain everywhere smooth. The latter feature is illustrated by examples including both equal and unequal mass BHs. This smoothness contrasts with the intrinsic geometry of the (spatial sections of the) horizon of these BHs, which is not smooth, and provides a sharp example on how BH shadows are insensitive to some horizon geometry details. This observation, moreover, suggests that for the study of their shadows, this static double BH system may be an informative proxy for a dynamical binary.

Testing the nature of dark compact objects: a status report

“…(9)], all studies done so far indicate that it is extremely challenging to use such an effect to place a constraint much stronger than Eq. (102) [91,461,462].…”

Black hole shadow in a general rotating spacetime obtained through Newman-Janis algorithm

“…Some recent studies, however, suggest that the presence of a shadow does not by itself prove that a compact object is necessarily a black hole. Other horizonless compact objects, which posses light rings around them, can also cast shadows [73][74][75][76][77][78][79][80][81][82][83][84][85][86][87][88].…”

On the shadow of rotating traversable wormholes

“…The effect was originally discovered when studying the light propagation in the vicinity of the Kerr black hole [1]. Yet, it is not restricted exclusively to the presence of an event horizon, but can be observed in more general geometries possessing a photon region, such as naked singularities, gravastars and wormholes [2][3][4].…”