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The optical appearance of a body compact enough to feature an unstable boundorbit, when surrounded by an accretion disk, is expected to be dominated by aluminous ring of radiation enclosing a central brightness depression typicallyknown as the shadow. Despite observational limitations, the rough details ofthis picture have been now confirmed by the results of the EHT Collaboration onthe imaging of the M87 and Milky Way supermassive central objects. However, theprecise characterization of both features - ring and shadow - depends on theinteraction between the background geometry and the accretion disk, thus beinga fertile playground to test our theories on the nature of compact objects andthe gravitational field itself in the strong-field regime. In this work we useboth features in order to test a continuous family of solutions interpolatingbetween regular black holes and horizonless compact objects, which arise withinthe Eddington-inspired Born-Infeld theory of gravity, a viable extension ofEinstein's General Relativity (GR). To this end we consider seven distinctiveclasses of such configurations (five black holes and two traversable wormholes)and study their optical appearances under illumination by a geometrically andoptically thin accretion disk, emitting monochromatically with three analyticintensity profiles previously suggested in the literature. We build such imagesand consider the sub-ring structure created by light rays crossing the diskmore than once and existing on top of the main ring of radiation. We discuss indetail the modifications as compared to their GR counterparts, the Lyapunovexponents of unstable nearly-bound orbits, as well as the differences betweenblack hole and traversable wormholes for the three intensity profiles.

Shadows and photon rings of regular black holes and geonic horizonless compact objects