Developing a physical model for the AGN UV/optical power spectra

The UV/optical emission of active galactic nuclei (AGN) has long been thought to be connected to their X-ray radiation due to X-ray reprocessing by the accretion disk. Recently, several Swift key projects were allocated to probe this connection by monitoring nearby AGN as part of long multi-wavelength campaigns. The results of these studies pointed at a discrepancy between observations and the underlying theory, as the X-ray and UV emission were found to be only mildly correlated and the detected time lags were larger than predicted by the standard accretion disk theory. In order to further examine the assumption of disc X-ray reprocessing, we have developed for the first time a physical model to reproduce the power spectra (PSDs) of AGN UV/optical light curves, by simulating the thermal reprocessing of the X-rays in the disk, while taking into account all the relativistic effects of photon propagation in the vicinity of a black hole. Applied to the long monitoring campaigns, this model offers a novel way to probe the inner region of AGN. We apply our model to a sample of sources (including NGC 5548) and we find that the X-ray and UV/optical PSDs as well as the observed UV/optical time lags are all consistently explained, while we also derive constraints on the source physical parameters, such as the accretion rate and the X-ray corona height. Our results suggest that X-ray disk reprocessing accounts for the full variability properties of the observed AGN emission, within the considered time scales, reconciling thus the observations with the theoretical predictions. Using earlier archival data, we further show that our model can also reproduce the PSD shape of NGC 5548 in different epochs, establishing the feasibility of using PSD fitting to study the evolution of a source. Finally, we demonstrate how the rapid variability of the X-ray source leads to low detected cross correlation between the X-rays and the disk emission, as has been observed in the recent literature.