A General Response Theory for Closure Phases through Visibility Nulls and Image Structure

Abstract

Closure phase is robust to station based phase errors, yet its connection to image structure is often interpreted only qualitatively and a nonzero closure phase is still interpreted mainly through qualitative symmetry arguments or source specific models. We develop, to our knowledge, the first unified response theory for an arbitrary brightness distribution and an arbitrary zero free observing triangle. Away from visibility zeros, closure phase is naturally a log-visibility observable. This structure yields an exact first order master kernel for image perturbations and a Cartesian moment hierarchy that separates the spatial content of the source change from the sensitivity of the sampled triangle. For a fixed image, the same framework gives a local expansion in Fourier plane displacement, providing a direct description of closure phase evolution along observing tracks. We then identify the boundary of this regular regime: at a true visibility null, the log-visibility expansion fails and the phase behavior is controlled by the local properties of the null, including its order and winding charge, distinguishing removable phase wraps, non-null visibility minima, and nulls for which the visibility phase is multivalued. We validate these expansions using structured ring models. We then apply both branches of the theory to EHT data of M87*, and demonstrate the ability to measure asymmetric image structure associated with higher image moments directly from the evolution of the closure phase. Finally, we show that the same formalism extends naturally to Phase Closure Nulling in optical and infrared interferometry. The resulting framework turns closure phase structure into a quantitative probe of faint and asymmetric source features across interferometric regimes.