Resolving the photon ring

In the past few years, the Event Horizon Telescope has released the first close-up interferometric images of two supermassive black holes, M87* and SgrA*. It is believed that within these images is embedded a fine, yet-unresolved brightness enhancement called the photon ring. The ring is a universal consequence of extreme lensing by the black hole and thereby conveys information on its spacetime geometry, potentially providing a new independent avenue for future tests of general relativity in the strong-field regime. In the talk I will briefly review the theory of the photon ring and its corresponding spacetime region, the photon shell, which governs the universal lensing structure. I will then describe some current efforts and future prospects for resolving the ring, which include both the construction of transformative new instruments and the development of novel analysis methods. Focusing on the latter, I will discuss how source variability may be harnessed to detect the ring. In particular I will review a recently proposed observable, the 2-point correlation function of intensity fluctuations around the ring, and present its recent generalization to frequency-dependent sources. Using a simple toy model of line-emitting, orbiting sources and integrating over the image, I will argue that extreme lensing effects induce particular spectro-temporal correlations in specific flux fluctuations that could be relevant for unresolved spectrometric observations