A physical model for the UV/optical power spectra of AGN

TL;DR

This paper presents a physical model that successfully reproduces the UV/optical variability power spectra of AGN by considering X-ray reprocessing in the accretion disk, providing insights into the innermost regions of AGN.

Contribution

The authors develop a novel physical model linking X-ray reprocessing to UV/optical variability, enabling detailed analysis of AGN inner structures and variability over time.

Findings

Model reproduces UV/optical PSDs of NGC 5548

Constraints on X-ray corona height and accretion rate

Model applicable to different epochs of the source

Abstract

The UV/optical variability of AGN has long been thought to be driven by the X-ray illumination of the accretion disk. However, recent multi-wavelength campaigns of nearby Seyfert galaxies seem to challenge this paradigm, with an apparent discrepancy between observations and the underlying theory. In order to further probe the connection between the UV/optical and X-ray variability in AGN we developed a physical model to reproduce the UV/optical power spectra (PSDs) of AGN assuming the thermal reprocessing of the X-rays in the disk. This model offers a novel way to probe the innermost regions of AGN. We use our model to study the variability of NGC 5548 and we infer that the X-ray and UV/optical PSDs as well as the interband UV/optical time lags are all well reproduced. We also derive constraints on the source physical parameters, such as the X-ray corona height and the accretion rate. Our results suggest that X-ray disk reprocessing accounts for the full variability properties of this AGN, within the considered time scales. Using earlier data of NGC 5548, we also show that our model can reproduce its PSD in different epochs, establishing the feasibility of using PSD modelling to investigate the time evolution of a source.