Light ring images of double photon spheres in black hole and wormhole space-times

TL;DR

This paper investigates light ring images of double photon spheres in black hole and wormhole spacetimes, showing that multiple critical curves can produce distinctive observational signatures, potentially distinguishing black holes from mimickers.

Contribution

It introduces a family of spherically symmetric geometries with double critical curves and demonstrates how additional light rings could serve as evidence for black hole mimickers.

Findings

Presence of additional light rings between two critical curves.

Observation of multiple critical curves could indicate black hole mimickers.

Distinctive light ring patterns depend on spacetime geometry.

Abstract

The silhouette of a black hole having a critical curve (an unstable bound photon orbit) when illuminated by an optically thin accretion disk whose emission is confined to the equatorial plane shows a distinctive central brightness depression (the shadow) whose outer edge consists of a series of strongly lensed, self-similar rings superimposed with the disk's direct emission. While the size and shape of the critical curve depend only on the background geometry, the pattern of bright and dark regions (including the size and depth of the shadow itself) in the image is strongly influenced by the (astro)physics of the accretion disk. This aspect makes it difficult to extract clean and clear observational discriminators between the Kerr black hole and other compact objects. In the presence of a second critical curve, however, observational differences become apparent. In this work we shall consider some spherically symmetric black hole and wormhole geometries characterized by the presence of a second critical curve, via a uniparametric family of extensions of the Schwarzschild metric. By assuming three toy models of geometrically thin accretion disks, we show the presence of additional light rings in the intermediate region between the two critical curves. The observation of such rings could represent a compelling evidence for the existence of black hole mimickers having multiple critical curves.