Higher-order inner photon rings of a horizonless ultracompact object with an antiphoton sphere and their interferometric pattern*

Abstract A horizonless ultracompact object can have a stable antiphoton sphere, which causes the photons inside the unstable photon sphere to strongly deflect, thereby leading to the formation of distinctive inner photon rings. In this study, we present analytical descriptions for the shape, thickness, and interference pattern of higher-order inner photon rings. By taking the static spherically symmetric Schwarzschild star with a photon sphere as an example, we find that its inner photon rings can be more non-circular and thicker than the outer ones and show that the inclusion of the inner photon rings can result in new features in the interferometric pattern. Further, our formulae can be applied to other ultracompact objects, providing a convenient approach for studying the observational properties of their higher-order photon rings.