Shadows of generalised Hayward spacetimes : in vacuum and with plasma

TL;DR

This paper explores the shadow features of generalized Hayward spacetimes, including black holes and wormholes, in vacuum and plasma, revealing potential observational signatures and constraints from Event Horizon Telescope data.

Contribution

It introduces a unified analysis of various Hayward-based spacetimes, including wormholes, and examines their shadow properties in different plasma environments.

Findings

Wormholes can exhibit multiple photon spheres, affecting shadow shapes.

Regular black holes are consistent with EHT observations within narrow parameters.

Wormholes with multi-peak potentials align better with shadow constraints than single-peak ones.

Abstract

We investigate the shadow properties of a wide class of spacetimes arising from different parameter regimes of the generalized Hayward metric, characterized by two independent parameters $(\sigma, \kappa)$ (Phys. Rev. D 106, 044028). This metric extends the original Hayward regular black hole solution by introducing distinct mass functions in the $g_{tt}$ and $g_{rr}$ components, giving rise to four types of wormholes ( which include multi-peak effective potentials), a regular black hole, and a singular black hole solutions allowing for a unified treatment of black hole mimickers. We compute the shadow radii for all spacetimes in vacuum and in the presence of plasma, using both homogeneous and non-homogeneous plasma profiles. Our results show that certain wormhole solutions particularly the Hayward-Damour-Solodukhin class can exhibit multiple photon spheres, leading to shadow features that differ significantly from the Schwarzschild black hole. When these results are compared with Event Horizon Telescope observations of Sgr~$A^\star$, we find that regular black holes remain observationally viable but only within a narrow parameter space. In contrast, wormhole solutions with multi-peak effective potentials are more consistent with shadow constraints than those with single peaks. This contrasts with quasinormal mode studies, which favored single-barrier potentials, and may imply detectable late-time echoes in gravitational wave signals.