Geometry of the Inner Shadow: Constraining Black Hole Spin, Mass, and Inclination

Images of black holes show striking characteristic features, such as a bright thin ring known as the photon ring and a central brightness depression which we call the inner shadow. Next generation very long baseline interferometry (VLBI), such as the next generation Event Horizon Telescope (ngEHT) and the Black Hole Explorer (BHEX), will deliver the dynamic range and angular resolution needed to produce sharp images of these features, introducing the possibility for a geometric analysis. In this talk, we investigate the inner shadow, which is the delineation between null geodesics that thread emitting matter and those that do not. We demonstrate that the position and shape of the inner shadow can be used to constrain parameters of the black hole, such as the spin, mass-to-distance ratio, and viewing inclination.