2018ApJ...869..114I

Central supermassive black holes of active galactic nuclei host hot plasma with a temperature of 10⁹ K, namely coronae. Like the Sun, black hole coronae are theoretically believed to be heated by their magnetic activity, which have not been observed yet. Here we report the detection of coronal radio synchrotron emission from two nearby Seyfert galaxies using the Atacama Large Millimeter/submillimeter Array, the Karl G. Jansky Very Large Array, and Australia Telescope Compact Array. The coronal magnetic field of both systems is estimated to be ∼10 G on scales of ∼40 Schwarzschild radii from the central black holes. This magnetic field strength is weaker than the prediction from the magnetically heated corona scenario. We also find that coronae of Seyferts are composed of both thermal and nonthermal electrons. This may imply a possible contribution of Seyferts to the cosmic MeV gamma-ray background radiation.