Shadow of regularized compact objects without a photon sphere

TL;DR

This paper explores how certain regularized spacetimes without a photon sphere can produce black hole shadows similar to those observed, challenging the necessity of a photon sphere for shadow formation.

Contribution

It demonstrates that shadows can form in regularized null singularity spacetimes without a photon sphere, using the Simpson-Visser regularization technique.

Findings

Shadows similar to Schwarzschild black holes can arise without a photon sphere.

SV regularization parameter affects shadow size and geometry.

Observational data constrains parameters of these spacetimes.

Abstract

Recent observations by the Event Horizon Telescope (EHT) indicate that the shadow of the compact object at our Galaxy's center (Sgr A*) closely resembles that of a Schwarzschild black hole. However, identifying the presence and exact location of a photon sphere observationally remains challenging. Motivated by this, we investigate shadow formation in spacetimes that lack a photon sphere by applying the Simpson-Visser (SV) regularization technique (originally designed to smooth black hole singularities) to null singularity and charged null singularity metrics. Remarkably, these regularized null and charged null singularity spacetimes can produce a shadow without a photon sphere. We analyze how the SV regularization parameter influences their geometry and shadow size, and show that the regularized null and charged null singularity spacetimes can correspond either to two-way traversable wormholes or retain singularities. Our results reveal that shadows arising from these regularized 'null singularity spacetimes' closely mimic those of Schwarzschild and charged black-bounce spacetimes, despite the absence of a photon sphere. We also constrain parameters in these geometries using observational data of Sgr A* and M87.