Detection of Coronal Magnetic Activity in Nearby Active Supermassive Black Holes

TL;DR

This study reports the first detection of coronal radio emission from nearby active supermassive black holes, revealing magnetic fields weaker than expected and a mix of thermal and non-thermal electrons, with implications for cosmic gamma-ray background.

Contribution

First observational evidence of coronal radio emission from supermassive black holes, measuring magnetic fields and electron composition.

Findings

Detected coronal radio synchrotron emission in Seyfert galaxies.

Estimated magnetic fields of ~10 Gauss at ~40 Schwarzschild radii.

Found coronae contain both thermal and non-thermal electrons.

Abstract

Central supermassive black holes of active galactic nuclei 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, which have never 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 $\sim10$ Gauss on scales of $\sim40$ 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 non-thermal electrons. This may imply a possible contribution of Seyferts to the cosmic MeV gamma-ray background radiation.