High-order Photon Rings around Kerr Naked Singularities

Abstract We present a detailed study of higher-order photon rings of an accreting flow in a overly spinning, horizonless, Kerr spacetime with dimensionless spin parameter a = 1.01. Motivated by horizon-scale very-long-baseline interferometry including Event Horizon Telescope and future missions such as the Black Hole Explorer, we analyze image morphology and interferometric visibilities to identify observational signatures that differentiate such a Kerr naked singularity from Kerr black holes. We find that higher-order photon rings are concentrated within the nominal “shadow” region and that the shadow develops a pronounced gap at sufficiently large observer inclination. These morphological differences produce measurable deviations in the complex visibilities relative to Kerr black hole predictions. Our results indicate that photon-ring structure and visibility-domain diagnostics at horizon-resolving baselines can provide a direct observational test of the presence (or absence) of an event horizon and thus offer a concrete avenue to test general relativity with future horizon-scale observations.