Modelling continuum reverberation in active galactic nuclei: a spectral-timing analysis of the ultraviolet variability through X-ray reverberation in Fairall 9

Continuum reverberation mapping of active galactic nuclei (AGNs) can provide new insight into the nature and geometry of the accretion flow. Some of the X-rays from the central corona irradiating the disc are absorbed, increasing the local disc temperature. This gives an additional reprocessed contribution to the spectral energy distribution (SED) which is lagged and smeared relative to the driving X-ray light curve. We directly calculate this reverberation from the accretion disc, creating fully time-dependent SEDs for a given X-ray light curve. We apply this to recent intensive monitoring data on Fairall 9, and find that it is not possible to produce the observed ultraviolet (UV) variability by X-ray reprocessing of the observed light curve from the disc. Instead, we find that the majority of the variability must be intrinsic to the UV emission process, adding to evidence from changing-look AGN that this region has a structure which is quite unlike a Shakura–Sunyaev disc. We filter out this long time-scale variability and find that reprocessing alone is still insufficient to explain even the fast variability in our assumed geometry of a central source illuminating a flat disc. The amplitude of reprocessing can be increased by any vertical structure such as the broad-line region and/or an inner disc wind, giving a better match. Fundamentally though the model is missing the major contributor to the variability, intrinsic to the UV/EUV emission rather than arising from reprocessing.