The Black Hole Photon Ring: Insights from GRMHD Simulations and Observational Prospects

The black hole photon ring exhibits a universal interferometric signature at long baselines, enabling a highly accurate measurement of its diameter. However, when confronted with real-world astrophysical and instrumental noises, the feasibility of such measurements with forthcoming space very-long-baseline interferometry (VLBI) missions remains unclear. In this talk, we present ray tracing calculations stemming from high-resolution general relativistic magnetohydrodynamic (GRMHD) simulations to elucidate the detectability of both the first and second photon rings over several years of observation. For M87*, our findings suggest that the first photon ring is detectable across all astrophysical models we considered, and similarly for the second photon ring provided that the emission is not excessively optically thick. On the other hand, discerning the photon ring signal becomes challenging in the transitional regime between the first and second photon rings, which we argue is due to the different scale dependence between the signal and astrophysical noise. Additionally, we propose a Bayesian framework to ensure empirically unbiased ring diameter estimation and to test the Kerr spacetime hypothesis.