Simulating X-ray Reverberation in the UV-Emitting Regions of Active Galactic Nuclei Accretion Disks with 3D Multi-Frequency Magnetohydrodynamic Simulations

TL;DR

This study uses 3D MHD simulations to explore how turbulence and X-ray irradiation influence UV variability in AGN accretion disks, finding turbulence drives UV variability independently of X-ray effects.

Contribution

It demonstrates that turbulence alone can produce UV variability consistent with observations, challenging the assumption that X-ray reprocessing is the primary driver.

Findings

Turbulence causes intrinsic UV variability with a good fit to a damped random walk model.

Injected X-rays have minimal impact on the UV light curve power spectrum.

X-ray and UV light curves are only correlated when X-ray variability occurs on timescales >1 day.

Abstract

Active galactic nuclei (AGN) light curves observed with different wavebands show that the variability in longer wavelength bands lags the variability in shorter wavelength bands. Measuring these lags, or reverberation mapping, is used to measure the radial temperature profile and extent of AGN disks, typically with a reprocessing model that assumes X-rays are the main driver of the variability in other wavelength bands. To demonstrate how this reprocessing works with realistic accretion disk structures, we use 3D local shearing box multi-frequency radiation magnetohydrodynamic (MHD) simulations to model the UV-emitting region of an AGN disk, which is unstable to the magnetorotational instability (MRI) and convection. At the same time, we inject hard X-rays ($>1$~keV) into the simulation box to study the effects of X-ray irradiation on the local properties of the turbulence and the resulting variability of the emitted UV light curve. We find that disk turbulence is sufficient to drive intrinsic variability in emitted UV light curves and that a damped random walk (DRW) model is a good fit to this UV light curve for timescales $>5$~days. Meanwhile, the injected X-rays have almost no impact on the power spectrum of the emitted UV light curve. In addition, the injected X-ray and emitted UV light curves are only correlated if there is X-ray variability on timescales $>1$~day, in which case we find a correlation coefficient $r=0.52$. These results suggest that hard X-rays with scattering dominated opacity are likely not the main driver of the reverberation signals.