Exploring Nonlinear Electrodynamics Theories: Shadows of Regular Black Holes and Horizonless Ultra-Compact Objects

TL;DR

This paper investigates how nonlinear electrodynamics (NED) theories influence the observable shadows of regular black holes and horizonless ultra-compact objects, proposing potential signatures detectable by the Event Horizon Telescope to test these theories.

Contribution

It provides the first detailed analysis of NED effects on black hole shadows and introduces observational signatures that can distinguish NED-based objects from traditional black holes.

Findings

NED-charged regular black holes have distinctive shadow features.

Horizonless ultra-compact objects show only one unstable photon ring.

NED signatures could be observed by the Event Horizon Telescope.

Abstract

In the Einstein-Maxwell theory with nonlinear electrodynamics (NED) fields, the singularity problem in general relativity is potentially resolved, leading to regular black hole solutions. In NED theories, photons follow null geodesics of an effective geometry that differs from the spacetime geometry itself. This raises an important question: Do NED fields produce unique observational signatures in the electromagnetic spectrum that can test regular black holes and NED theories? We analyze the shadows of two NED-charged regular black holes and their horizonless ultracompact objects (HUCOs) under two accretion models, comparing them with Schwarzschild black holes, focusing on shadow size, central brightness depression, and photon ring characteristics. Our results identify distinctive NED signatures that could be observable by the EHT, providing empirical evidence of NED fields and potentially ruling out previously considered viable candidates for astrophysical black holes models based on shadow measurements. Notably, NED-charged HUCOs generally exhibit only one $\textit{unstable}$ photon ring, thus avoiding the dynamical instability associated with stable photon rings and challenging the idea that objects with photon rings must be black holes.