Probing Kerr spacetime via interferometric signatures of higher-order images

Abstract

Higher-order images produced by photons that orbit close to the unstable photon region of a black hole encode information about the spacetime geometry and leave characteristic signatures accessible with long interferometric baselines. We develop an analytical description of these signatures for a compact source in the spacetime of a slowly rotating Kerr black hole. Using the strong deflection limit, we derive the complex visibility function for a source of finite size at linear order in the black-hole spin. For a static equatorial source, we obtain a direct relation between the dimensionless Kerr spin parameter and the angular displacement of successive higher-order images belonging to the same parity sequence. We then extend the construction to a compact source on a circular equatorial orbit by evaluating each image contribution at the corresponding retarded source position. Frame dragging and source motion modify the angular positions of the images and leave distinct imprints on the long-baseline visibility pattern. For the configuration considered here, these effects act in opposite angular directions, so that the resulting structure is determined by their competition. Our results provide an analytical framework for identifying the imprints of black-hole spin and source dynamics in the interferometric signatures of higher-order images.