Using Observations of Black-Hole Images to Probe Plasmas in Extreme Gravitational Fields

The ring-like images of the two supermassive black holes captured by the Event Horizon Telescope (EHT) provide powerful probes of the physics of accretion-flow plasmas in the extreme gravitational fields surrounding black holes. Specifically, the brightness asymmetry in the images carries information about the angular velocity profile of the inner accretion flow, owing to the Doppler boosts photons experience at their site of emission. In this talk, I will show that, in order to accommodate the lack of significant brightness asymmetry in the observed images of the black hole at the center of the Milky Way, Sgr A*, the plasma velocities must be significantly sub-Keplerian, i.e., non geodesic, and the black-hole spin cannot be large. Imaging the black hole at different wavelengths with future observations will provide strong constraints not only on the dynamics but also on the emission properties (and hence the thermodynamics) of the plasma in the same strong-field regions.