General relativity predicts that gravitational lensing near black holes will produce narrow "photon rings" on images. Building on recent work of Johnson, Lupsasca et al. focusing on circular rings, I calculate the long-baseline interferometric signature of a narrow feature of arbitrary shape and intensity. The shape information is contained in the oscillation frequencies of the complex visibility as a function of baseline length, for each baseline angle. These results raise the possibility of measuring the detailed shape of a photon ring and comparing to the precise predictions of general relativity.
@article{arxiv.2005.03856,
title = {Measuring the shape of a black hole photon ring},
author = {Samuel E. Gralla},
journal= {arXiv preprint arXiv:2005.03856},
year = {2020}
}