Simultaneous radio, optical, and X-ray monitoring of hard X-ray-selected active galactic nuclei: A variability study

Active galactic nucleus (AGN) emission is intrinsically variable across the electromagnetic spectrum. Mapping the coupling between the accretion disk, the X-ray corona, and ejection flows is key to understanding the energy flow within the central engine. We aim to characterize the multiwavelength variability of hard X-ray selected AGNs across the radio, optical, and X-ray bands. Our goal is to determine the degree of coupling between these frequencies and to explore how variability features are related to the physical properties of the central engine, with an emphasis on the radio band. The sample consists of 14 AGN-selected from the /IBIS catalog. We analyzed multi-epoch observations from the Arcminute Microkelvin Imager (AMI-LA) at 15 GHz, the Zwicky Transient Facility (ZTF) in the g and r bands, and /XRT, covering a two-year monitoring period (2018--2020). The variability was quantified using the normalized excess variance, the fractional variability amplitude, and the Mexican hat filter at 70- and 200-day timescales. Additionally, we characterized the radio loudness of the sample using both X-ray and optical definitions, then evaluated the impact of variability on the fundamental plane (FP) of black hole activity by comparing results from time-averaged data against strictly simultaneous observations. Significant variability was detected in 86% of the sample, displaying a clear amplitude stratification across the spectrum. The fractional root mean square variability amplitude is highest in X-rays with a median of 30% (11--67%), followed by the optical g and r bands at 19% (2--33%) and 8.5% (0.2--24%), and the radio band at 10% (4--23%). The Mexican hat analysis reveals a red-noise power spectrum where long-term fluctuations dominate. The sample follows the expected scaling of the FP where, despite individual sources shifting within the relation due to stochastic fluctuations, such variability-induced dispersion accounts for only ∼3% of the total scatter. Our findings support a core-dominated origin for the 15 GHz emission, likely arising from a compact jet base or a magnetized corona. These results also reflect the intrinsic physical diversity across these AGN cores, where differences in variability patterns, radio loudness, and the FP location point toward distinct accretion-and-ejection processes as well as varying degrees of corona-jet coupling. INTEGRAL Swift