2025MNRAS.544.1012H

The accretion flow in active galactic nuclei (AGN) is not well understood, motivating intensive monitoring campaigns of multiwavelength variability to probe its structure. One of the best of these is the 3 yr optical/UV/X-ray approximately daily monitoring campaign on Fairall 9, a fairly typical moderate accretion rate AGN. The UV light curve shows a clear increase over $\sim 50$ d between years 1 and 2, strongly coherent with the X-ray light curve rise. This changes the average spectral energy distribution (SED) such that the disc component is stronger while the X-ray spectrum steepens, so that the total X-ray power remains roughly constant. Outside of this global change, we apply a Fourier-resolved analysis to test stochastic models where intrinsic fluctuations in the UV disc propagate down into the hard X-ray emission region via both changing the seed photon flux for Compton scattering (short light travel time-scale) and changing the electron density (longer propagation time-scale). Unlike these models, the hard X-rays are not particularly well correlated with the UV, and also have the wrong sign in that the hard X-rays marginally lead the UV fluctuations. We show that this is instead consistent with uncorrelated stochastic fluctuations in both the UV (slow) and X-ray (fast), which are linked together only weakly via light travel time. These variability properties, as well as the changes in the SED, have implications for our understanding of AGN structure and physics, as well as future monitoring campaigns.