Joint ALMA/X-ray monitoring of the radio-quiet type 1 active galactic nucleus IC 4329A

Abstract

The origin of a compact millimeter (mm, 100--250 GHz) emission in radio-quiet active galactic nuclei (RQ AGN) remains debated. Recent studies propose a connection with self-absorbed synchrotron emission from the accretion disk X-ray corona. We present the first joint ALMA (sim 100 GHz) and X-ray ( 2--10\,keV) observations of the unobscured RQ AGN, IC 4329A ($z = 0.016$). The time-averaged mm-to-X-ray flux ratio aligns with recently established trends for larger samples, but with a tighter scatter (sim 0.1 dex) compared to previous studies. However, there is no significant correlation on timescales of less than 20 days. The compact mm emission exhibits a spectral index of $-0.23 0.18$, remains unresolved with a 13 pc upper limit, and shows no jet signatures. Notably, the mm flux density varies significantly (by factor of 3) within four days, exceeding the contemporaneous X-ray variability and showing the largest mm variations ever detected in RQ AGN over daily timescales. The high amplitude variability rules out scenarios of heated dust and thermal free--free emission, pointing toward a synchrotron origin for the mm radiation in a source of sim 1 light day (sim 120 gravitational radii) size. While the exact source is not yet certain, an X-ray corona scenario emerges as the most plausible compared to a scaled-down jet or outflow-driven shocks.