Gravitational lensing and ghost images in the regular Bardeen no-horizon spacetimes

TL;DR

This paper investigates gravitational lensing and unique optical effects, including ghost images, in the regular Bardeen no-horizon spacetimes, highlighting phenomena absent in black hole or naked singularity spacetimes.

Contribution

It introduces the concept of ghost images caused by low impact parameter photons and analyzes their properties in Bardeen no-horizon spacetimes, providing new insights into their optical signatures.

Findings

Existence of a critical impact parameter for maximal light deflection.

Ghost images occur only at large observer inclination angles.

Distinct frequency shift ranges differentiate ghost images from direct images.

Abstract

We study deflection of light rays and gravitational lensing in the regular Bardeen no-horizon spacetimes. Flatness of these spacetimes in the central region implies existence of interesting optical effects related to photons crossing the gravitational field of the no-horizon spacetimes with low impact parameters. These effects occur due to existence of a critical impact parameter giving maximal deflection of light rays in the Bardeen no-horizon spacetimes. We give the critical impact parameter in dependence on the specific charge of the spacetimes, and discuss "ghost" direct images of Keplerian discs, generated by photons with low impact parameters. The ghost images can occur only for large inclination angles of distant observers. We determine the range of the frequency shift of photons genering the ghost images and determine distribution of the frequency shift accross these images. We compare them to those of the standard direct images of the Keplerian discs. The difference of the ranges of the frequency shift on the ghost and direct images could serve as a quantitative measure of the Bardeen no-horizon spacetimes. The regions of the Keplerian discs giving the ghost images are determined in dependence on the specific charge of the no-horizon spacetimes. We can conclude that the optical effects related to the low impact parameter photons give clear signatures of the regular Bardeen no-horizon spacetimes, as no similar phenomena could occur in the black hole or naked singularity spacetimes. Similar phenomena have to occur in any regular no-horizon spacetimes having nearly flat central region.