Measuring Spin from Relative Photon-ring Sizes

The direct detection of a bright, ring-like structure in horizon-resolving images M87* by the Event Horizon Telescope is striking validation general relativity. angular size and shape ring degenerate measure location emission region, mass, spin black hole. However, we show that observation multiple rings, corresponding to low-order photon can break this degeneracy produce mass measurements independent rings. We describe two potential experiments would spin. In first, observations $n=1$ are made at epochs with different locations. This method conceptually similar spacetime constraints arise from variable structures (or hot spots) it breaks near-perfect between location, for polar observers using temporal variability. second, emission, $n=2$ rings during single epoch. For both schemes, additional comprise test Thus, comparisons 2017 2018 may be capable producing first horizon-scale estimates inferred strong lensing alone. Additional campaigns future high-frequency, Earth-sized space-based radio interferometers high-precision tests