X-ray disc reverberation modelling of the X-ray/UV/optical spectral/timing properties of Fairall 9
Multiwavelength monitoring surveys of active galactic nuclei (AGN) have revealed correlated variability observed in the X-ray, UV, and optical bands. X-ray reverberation, arising from the absorption of X-rays illuminating the accretion disc, provides a self-consistent physical framework for interpreting these observations and imposing constraints on the geometry and energetics of accretion flows and X-ray coronae. We aim to apply the X-ray disc reverberation framework to the Seyfert 1 galaxy Fairall 9, a well-studied AGN with a clear line of sight to the accretion disc, to investigate whether this physical scenario can simultaneously account for its observed spectral and timing properties, as probed by its mean spectral energy distribution (SED), UV/optical power spectral densities (PSDs), and interband time lags. We used multiwavelength data from the 2018-2021 Swift intensive monitoring campaign to construct the mean X-ray/UV/optical SED and to compute PSDs in all bands. We first modelled the broadband average SED using KYNSED, which is a physical X-ray reverberation model assuming lamp-post geometry. The resulting best-fit parameter space was then used to model the UV/optical PSDs and further constrain the physical parameters of the system. Finally, we tested whether the observed interband time lags are consistent with the model predictions for the parameter sets that simultaneously reproduce both the SED and the PSDs. X-ray illumination of the accretion disc can explain the broadband mean SED of Fairall 9. The UV/optical variations are likely driven by the variable X-rays that illuminate the disc, and not by short-timescale disc fluctuations of unknown physical origin. X-ray disc illumination and reverberation can explain the mean energy spectrum, the UV/optical power spectra, and the wavelength-dependent time lags simultaneously for a common set of physical parameters.