Horizon replicas in black hole shadows

TL;DR

This paper introduces horizon replicas, special null orbits related to black hole shadows, which could help determine black hole spins from observational data, advancing understanding of black hole physics.

Contribution

It presents a new analysis of black hole shadow boundaries using horizon replicas, linking photon orbits to black hole spin measurements.

Findings

Horizon replicas are related to specific photon orbits on the shadow boundary.

Conditions for imprinting horizon replicas depend on black hole spin and viewing angle.

New templates for black hole shadow observations are proposed.

Abstract

Recently, new exploratory channels have opened up for the physics of highly compact objects, such as gravitational waves and black hole shadows. Moreover, more precise analysis and observations are now possible in the physics of accretion around compact objects. These advancements provide in particular an unprecedented insight into the physics near the horizons of a black hole. In this work we focus on the shadow boundary of a Kerr black hole, introducing observables related to special null orbits, called horizons replicas, solutions of the shadow edge equations which are related to particular photon orbits, defined by constraints on their impact parameter, carrying information about the angular momentum of the central spinning object. These orbits are related to particular regions on the shadow boundary and might be used to determine the spin of the black hole. The results provide the conditions by which horizon replicas are imprinted in the black hole shadow profile, in dependence on the black hole dimensionless spin and observational angle, providing eventually new templates for the future observations.