Interferometric Signatures of Black Holes with Multiple Photon Spheres

It has been reported that the photon ring structure in black hole imagesproduces strong and universal interferometric signatures on longinterferometric baselines, holding promise for measuring black hole parametersand testing general relativity. This paper investigates the interferometricsignatures of black holes with one or two photon spheres, specifically withinthe framework of Einstein-Maxwell-Scalar models. Notably, for black holespossessing two photon spheres, interference between light rays orbiting theinner and outer photon spheres manifests as beat signals in the visibilityamplitude, deviating from the universal signatures observed in thesingle-photon sphere case.