The Fingerprints of Black Holes - Shadows and their Degeneracies

TL;DR

This paper demonstrates that black hole shadows can uniquely determine key parameters like angular momentum, charge, and observer position in Kerr-Newman spacetimes, but not in more complex Kerr-Newman-Taub-NUT cases due to degeneracies.

Contribution

It establishes the conditions under which black hole shadows uniquely identify black hole parameters and highlights degeneracies in more complex spacetimes.

Findings

Observers can determine black hole parameters from shadows in Kerr-Newman spacetime.

Degeneracies exist in Kerr-Newman-Taub-NUT spacetime, preventing full parameter resolution.

Black hole shadows encode information about observer position and velocity.

Abstract

We show that, away from the axis of symmetry, no continuous degeneration exists between the shadows of observers at any point in the exterior region of any Kerr-Newman black hole spacetime of unit mass. Therefore, except possibly for discrete changes, an observer can, by measuring the black holes shadow, determine the angular momentum and the charge of the black hole under observation, as well as the observer's radial position and angle of elevation above the equatorial plane. Furthermore, his/her relative velocity compared to a standard observer can also be measured. On the other hand, the black hole shadow does not allow for a full parameter resolution in the case of a Kerr-Newman-Taub-NUT black hole, as a continuous degeneration relating specific angular momentum, electric charge, NUT charge and elevation angle exists in this case.