Magnetic fields have long been thought to play key roles in driving accretion onto black holes and launching their relativistic jets. I will present spatially resolved long-baseline interferometry observations of linearly polarized synchrotron radiation from near the event horizons of two weakly accreting supermassive black holes, Sgr A* and M87. I will show how the data can be used to infer properties of the radiating plasma, and map the magnetic field structure. The data imply the presence of dynamically important magnetic fields in the emission region in both cases, which can alter the structure of the accretion flow and launch powerful jets.
Host: Erin Kara