Reproducing the UV/optical variability of AGN

AGN reverberation mapping is based on the broadly accepted idea that the X-ray corona irradiates the accretion disk, causing a delayed UV/optical response. In recent years, several Swift Key Projects were allocated to probe the X-ray/UV connection by monitoring nearby AGN as part of long multi-wavelength campaigns. However, all of these studies show a low X-ray/UV correlation and larger than expected interband time lags. In this talk, I will discuss a simple solution to these major outstanding puzzles in the field. First, in Panagiotou et al. 2022a, we developed the first physical model to fit the UV/optical power spectra of AGN, assuming thermal reprocessing of the X-rays in the disk and considering all the relativistic effects of the process. We apply our model to a sample of sources and we found that the observed power spectra and time lags can be well explained. Expanding on this result, in Panagiotou et al. 2022b, we show that the low X-ray/UV correlation is, in fact, expected if the corona is dynamic. This is simply due to the underlying assumptions in the definition of the cross correlation function statistic. Our results suggest that X-ray disk reprocessing accounts for the full variability properties of the observed AGN emission, reconciling observations with theoretical predictions; while our model offers a novel way to probe the inner region of AGN.