Abstract:
Central supermassive black holes of active galactic nuclei (AGNs) host hot plasma with a temperature of 10^{^9} K, namely coronae. Like the Sun, black hole coronae are theoretically believed to be heated by their magnetic activity. However, such activity has not been observed yet. In this talk, I will report the first clear detection of coronal magnetic activity in nearby AGNs using ALMA (radio telescopes). The measured magnetic field is weaker than the theoretical expectation, requiring a new corona heating mechanism. I will also discuss particle acceleration processes in the coronae of Seyferts based on our ALMA measurements. Such processes would produce high energy neutrinos seen by IceCube and MeV gamma-ray photons. Future radio (ALMA, ngVLA, LST), X-ray (XRISM, FORCE, Athena), and MeV gamma-ray (COSI, GRAMS, AMEGO) observations will provide critical tests to our scenario.