Millimeter Emission from Coronae in Radio-Quiet AGN

Millimeter (mm) emission from the nuclear regions of active galactic nuclei (AGN) offers a powerful probe of the innermost environments surrounding supermassive black holes (SMBHs). In this talk, I will present recent results investigating the physical origin of this emission, most likely arising from the hot X-ray-emitting corona, and discuss what it reveals about the region in immediate vicinity of the SMBH through two complementary projects.
The first investigates the correlation between mm and X-ray emission in luminous, radio-quiet (RQ) AGN. Previous work has shown a tight mm/X-ray relation for low-luminosity AGN, consistent with a common origin in the hot X-ray corona. Using quasi-simultaneous ALMA and Swift observations of nine luminous AGN from the BASS survey, we find that these sources lie above the established relation, showing systematically higher mm/X-ray ratios. The mm emission also correlates linearly with bolometric luminosity but not with UV emission, suggesting that it primarily traces the total accretion power. These results indicate that either the relative X-ray output decreases at higher accretion rates or additional components, such as mm emission from AGN-driven outflows, contribute to the observed mm flux.
The second project focuses on the nature of coronal synchrotron emission in Seyfert galaxies. Using ALMA Bands 3, 4 and 6, we built broadband mm spectra for nine AGN to search for signatures of synchrotron emission and variability. The measured spectral indices and short-timescale variability imply compact, optically thin synchrotron emission consistent with a coronal origin. Future modeling with radially stratified corona models will allow us to constrain key parameters such as magnetic field strength and electron density.
Together, these studies show that mm emission offers a new observational window into the physics of AGN coronae and magnetic fields.