Shadow of a dressed black hole and determination of spin and viewing angle

TL;DR

This paper extends shadow characterization methods to dressed black holes with accretion disks, enabling estimation of spin and viewing angle, but finds it challenging to distinguish Kerr from non-Kerr black holes based solely on shadow shape.

Contribution

The study adapts the Hioki-Maeda shadow analysis to black holes with accretion disks, exploring parameter inference and the difficulty in testing the Kerr metric.

Findings

Shadow shape depends strongly on spin and inclination.

Shape alone cannot reliably distinguish Kerr from non-Kerr black holes.

Method provides a way to estimate black hole parameters from shadow observations.

Abstract

Shadows of black holes surrounded by an optically thin emitting medium have been extensively discussed in the literature. The Hioki-Maeda algorithm is a simple recipe to characterize the shape of these shadows and determine the parameters of the system. Here we extend their idea to the case of a dressed black hole, namely a black hole surrounded by a geometrically thin and optically thick accretion disk. While the boundary of the shadow of black holes surrounded by an optically thin emitting medium corresponds to the apparent photon capture sphere, that of dressed black holes corresponds to the apparent image of the innermost stable circular orbit. Even in this case, we can characterize the shape of the shadow and infer the black hole spin and viewing angle. The shape and the size of the shadow of a dressed black hole are strongly affected by the black hole spin and inclination angle. Despite that, it seems that we cannot extract any additional information from it. Here we study the possibility of testing the Kerr metric. Even with the full knowledge of the boundary of the shadow, those of Kerr and non-Kerr black holes are very similar and it is eventually very difficult to distinguish the two cases.