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

Light-ring bifurcations that can occur for prolate non-Kerr compact objects can leave an indelible signature on supermassive black hole 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—toward either 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.