Discovery of millimetre-wave excess emission in radio-quiet active galactic nuclei

The physical origin of radio emission in radio-quiet active galactic nuclei (RQ AGN) remains unclear, whether it is a downscaled version of the relativistic jets typical of radio-loud (RL) AGN, or whether it originates from the accretion disc. The correlation between 5 GHz and X-ray luminosities of RQ AGN, which follows L_R = 10^-5L_X observed also in stellar coronae, suggests an association of both X-ray and radio sources with the accretion disc corona. Observing RQ AGN at higher (mm-wave) frequencies, where synchrotron self-absorption is diminished, and smaller regions can be probed, is key to exploring this association. Eight RQ AGN, selected based on their high X-ray brightness and variability, were observed at 95 GHz with the CARMA (Combined Array for Research in Millimetre-wave Astronomy) and ATCA (the Australia Telescope Compact Array) telescopes. All targets were detected at the 1-10 mJy level. Emission excess at 95 GHz of up to ×7 is found with respect to archival low-frequency steep spectra, suggesting a compact, optically thick core superimposed on the more extended structures that dominate at low frequencies. Though unresolved, the 95 GHz fluxes imply optically thick source sizes of 10^-4-10^-3 pc, or ~10-1000 gravitational radii. The present sources lie tightly along an L_R (95 GHz) = 10^-4L_X (2-10 keV) correlation, analogous to that of stellar coronae and RQ AGN at 5 GHz, while RL AGN are shown to have higher L_R/L_X ratios. The present observations argue that simultaneous mm-wave and X-ray monitoring of RQ AGN features a promising method for understanding accretion disc coronal emission.