Unveiling the Nature of Coronae in Active Galactic Nuclei through Sub-mm Observations

TL;DR

This paper explores how sub-millimeter observations with ALMA can detect synchrotron radiation from magnetized coronae in active galactic nuclei, revealing magnetic fields and non-thermal electrons.

Contribution

It predicts observable signatures of magnetized coronae in AGNs at sub-mm wavelengths, especially the non-thermal synchrotron tail detectable by ALMA.

Findings

Thermal electrons produce flux below ALMA sensitivity.

Non-thermal electrons generate detectable synchrotron emission.

Future ALMA observations can probe coronal magnetic fields.

Abstract

The heating mechanism of a corona above an accretion disk in active galactic nuclei (AGNs) is still unknown. One possible mechanism is magnetic reconnection heating requiring energy equipartition between magnetic energy and gas energy in the disk. Here, we investigate the expected observed properties in radio band from such a magnetized corona. A magnetized corona can generate synchrotron radiation since a huge amount of electrons exists. Although most of radiation would be absorbed by synchrotron self-absorption, high-frequency end of synchrotron emission can escape from a corona and appears at the sub-mm range. If only thermal electrons exist in a corona, the expected flux from nearby Seyferts is below the Atacama Large Millimeter/ submillimeter Array (ALMA) sensitivity. However, if non-thermal electrons coexist in a corona, ALMA can measure the non-thermal tail of the synchrotron radiation from a corona. Such non-thermal population is naturally expected to exist if the corona is heated by magnetic reconnections. Future ALMA observations will directly probe the coronal magnetic field strength and the existence of non-thermal electrons in coronae of AGNs.