Fractal signatures of non-Kerr spacetimes in the shadow of light-ring bifurcations

TL;DR

This paper explores how non-Kerr spacetimes with light-ring bifurcations produce fractal shadow features, which could serve as observational signatures distinguishing them from standard Kerr black holes.

Contribution

It demonstrates that prolate non-Kerr objects generate fractal shadow structures due to photon resonance effects, providing potential observable indicators of non-Kerr geometries.

Findings

Fractal eyebrow-like features appear in shadows of non-Kerr spacetimes.

Multiple photon escape routes create complex shadow patterns.

Fractal structures are likely observable in SMBH shadows with specific orientations.

Abstract

Light-ring bifurcations that can occur for prolate non-Kerr compact objects can leave an indelible signature on SMBH shadows as a fractal sequence of eyebrow-like formations. These fractal features are the result of two properties of these spacetimes. The first is that they allow for multiple escapes for the photons (throats in the effective potential of photon geodesic motion). The second is that photon geodesics can resonate between different generalized light-rings related to the escapes, called fundamental photon orbits, that lead photons to alternate between the different exits either towards the compact object or infinity. The resulting fractal structures of the shadow seem to be a generic feature of prolate non-Kerr objects that may be observable in (accretion-disk)-illuminated compact objects, especially along equatorial lines of sight, but the best orientation depends on the specific parameters. Such fractal features if observed in the shadows of singular supermassive black holes at the centers of galaxies, would be smoking gun signals of non-Kerr compact objects.