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

TL;DR

This study uses advanced simulations to explore how X-ray irradiation affects AGN accretion disks, revealing conditions for effective reprocessing and implications for observed variability and lag measurements.

Contribution

The paper introduces 3D radiation magnetohydrodynamic simulations with detailed opacity models to analyze X-ray reprocessing in AGN disks, highlighting key factors influencing UV-X-ray correlation.

Findings

X-ray to UV luminosity ratio and absorption determine reprocessing efficiency.

X-ray heating can significantly increase disk temperatures.

Linear models can recover lags despite complex reprocessing processes.

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.