Continuum Reverberation in Active Galactic Nuclei Disks Only With Sufficient X-ray Luminosity and Low Albedo

Abstract

Disk continuum reverberation mapping is one of the primary ways we learn about active galactic nuclei (AGN) accretion disks. Reverberation mapping assumes that time-varying X-rays incident on the accretion disk drive variability in UV-optical light curves emitted by AGN disks, and uses lags between X-ray and UV-optical variability on the light-crossing timescale to measure the radial temperature profile and extent of AGN disks. However, recent reverberation mapping campaigns have revealed oddities in some sources such as weakly correlated X-ray and UV light curves, longer than anticipated lags, and evidence of intrinsic variability from disk fluctuations. To understand how X-ray reverberation works with realistic accretion disk structures, we perform 3D multi-frequency radiation magnetohydrodynamic simulations of X-ray reprocessing by the UV-emitting region of an AGN disk using sophisticated opacity models that include line opacities for both the X-ray and UV radiation. We find there are two important factors that determine whether X-ray irradiation and UV emission will be well-correlated, the ratio of X-ray to UV luminosity and significant absorption. When these factors are met, the reprocessing of X-rays into UV is nearly instantaneous, as is often assumed, although linear reprocessing models are insufficient to fully capture X-ray reprocessing in our simulations. Nevertheless, we can still easily recover mock lags in our light curves using software that assumes linear reprocessing. Finally, the X-rays in our simulation heat the disk, increasing temperatures by a factor of 2--5 in the optically thin region, which could help explain the discrepancy between measured and anticipated lags.