Light rings and causality for nonsingular ultracompact objects sourced by nonlinear electrodynamics

TL;DR

This paper investigates how nonlinear electrodynamics influences light propagation around nonsingular ultracompact objects, revealing unique light ring structures and potential causality issues, with implications for observational signatures.

Contribution

It introduces the concept of birefringence-induced light rings in nonsingular ultracompact objects and analyzes their observational and causal properties.

Findings

Nonsingular ultracompact objects have an odd number of light rings.

Birefringence causes photons of different polarizations to follow distinct paths.

Regularization via nonlinear electrodynamics can lead to acausal regions.

Abstract

We study observational signatures of nonsingular ultracompact objects regularized by nonlinear electrodynamics. The phenomenon of birefringence causes photons of different polarizations to propagate with respect to two distinct metrics, which manifests itself in the appearance of additional light rings surrounding the ultracompact object. We analyze the observational consequences of this result and illustrate our findings based on three regular black hole models commonly considered in the literature. We find that nonsingular horizonless ultracompact objects sourced by nonlinear electrodynamics possess an odd number of light rings and discuss the viability of this model as an effective description of their properties. In addition, we compare the phase velocities of polarized light rays propagating in nonsingular geometries sourced by nonlinear electrodynamics to the corresponding phase velocity in the Schwarzschild spacetime and demonstrate that regularizing the singularity by means of a theory that does not adhere to the Maxwell weak-field limit may lead to the emergence of acausal regions.