Disk Structure May Determine AGN Variability and Explain the Accretion Disk Size Problem: No Broad Line Region Required

The gist: 3D multi-frequency radiation magnetohydrodynamic simulations of an AGN disk reveal that disk structure, specifically whether it is thin or thick, determines its variability and can explain discrepancies between traditional X-ray-driven models and recent observations.

Simulation Setup and Disk Structure

The study performs 3D multi-frequency radiation magnetohydrodynamic simulations of the 1300 − 5940 ˚A emitting region of an AGN disk, which stochastically has different disk thickness above and below the midplane The simulations are configured as a 3D wedge with a broad radial range of (r, θ, ϕ) ∈ [50 rg, 103 rg]×[0, π]×[0, 0.39] to resolve the inner disk and the disk mid-plane The simulations are initialized with a single magnetic-field loop and become magnetically dominated during the early evolution The structure of the bottom of the disk is different from that of the top of the disk, which has a large impact on the light curves emitted from the bottom of the disk The top of the disk is thin, with a UV-optical photosphere at z ≲ 25 rg at all radii The bottom of the disk exhibits a puffed up structure driven by convection arising from entropy inversions within the UV-optical photosphere below the disk midplane

Variability Drivers and Light Curve Analysis

The simulations show that light curves emitted from the thin top of the disk are broadly consistent with traditional variability models, where X-ray irradiation is the main driver of variability in UV-optical light curves Conversely, the UV-optical light curves emitted from the thicker bottom of the disk are not strongly correlated with the X-ray light curve and have disk continuum reverberation mapping lags that are 3 − 5× longer than the light travel time The simulations allow for a new model where a thicker disk leads to greater X-ray absorption, less X-ray reprocessing at larger disk radii, and a stronger signal from intrinsic variability driven by fluctuations in the AGN disk Intrinsic variability in UV-optical light curves is shown to arise from fluctuations in the UV-optical emitting region of the disk due to magnetorotational instability (MRI) driven turbulence or convection

Comparison with Traditional Models

The traditional model predicts that X-ray variability drives UV and optical variability, leading to lags consistent with the light-crossing timescale The light curves emitted from the tops of simulated disks are highly coherent and correlated with the injected X-ray light curve, with lags consistent with the light-crossing timescale However, for the bottom of the disk, variability is inconsistent with traditional RM models The bottom of the disk shows longer than anticipated continuum lags, which is not consistent with traditional RM models

Long Lags and Inflow Timescale

The simulations reveal long lags on the inflow timescale as a result of propagating density fluctuations driven by MRI and convection The propagation speeds of these fluctuations are roughly consistent with the averaged inflow speeds in Figure 14, suggesting that temperature fluctuations inferred from broad band photometry could probe this inflow timescale Long lags between different radii light curves can be measured using cross-correlation and Fourier methods, suggesting an inflow speed on the order of 107−108 cm s−1

Disk Thickness Estimation

The differences between the light curves emitted from the thin top of the disk and the thick bottom of the disk provide a method to estimate disk thickness The accretion rate for the thick bottom of the disk is nearly double that for the thin top of the disk at inner boundary radii, suggesting that accretion rate can be correlated with disk thickness

Conclusion

Thin AGN disks with sufficient X-ray luminosity are consistent with traditional RM models Thicker disks and lower X-ray luminosity are inconsistent with traditional RM models, but in line with recent observations of moderate to weakly correlated X-ray and UV light curves Fluctuations in gas density driven by MRI turbulence and convection produce intrinsic variability on the disk’s thermal timescales Long lags between different radii light curves can be measured using cross-correlation and Fourier methods, suggesting an inflow speed on the order of 107−108 cm s−1 In addition to measuring long lags, we can use the differences we see in light curves emitted from the thin and thick sides of our simulated disks to estimate the thickness of observed AGN disks using RM and the PSDs of their light curves Improved models for AGN light curves are crucial for preparing for the Rubin Observatory, a high-cadence and long baseline survey that will observe up to millions of AGN

Caveats

The simulations rely on assumptions such as a fixed axisymmetric corona and a single power-law PSD to model X-ray variability Future work should test different X-ray PSDs to examine the impact they have on emitted light curves in non-shearing box simulations The paper also notes that the disk structure can change significantly over longer timescales of the original gray simulation

Conclusion

The center for computational astrophysics at the flatiron institute is supported by the simons foundation The simulations reveal a unified picture of density fluctuations driven by the MRI and convection propagating on the inflow speed producing identifiable lag signals as a function of radius in the emitted light curves Measuring the inflow timescale of the disk through the observation of long lags would allow us to test the standard thin disk model and probe the vertical structure of the disk, because the inflow timescale depends on the disk aspect ratio The simulations reveal that both disk structures are possible for these AGN parameters but arise stochastically Future RM studies can incorporate this intrinsic variability into their mock light curves using the PSDs in figures 5 and 8 Improved models for AGN light curves are crucial for preparing for the rubin observatory, a high-cadence and long baseline survey that will observe up to millions of agn The center for computational astrophysics at the flatiron institute is supported by the simons foundation

How it works

1. 3D multi-frequency radiation magnetohydrodynamic simulations are performed of an AGN disk irradiated by time-varying X-rays modeled as if they are emitted by an AGN corona

2. The light curves emitted from the thin top of the disk are consistent with traditional X-ray-driven RM models, especially for runs with higher X-ray luminosities

3. For the bottom of the disk, variability is inconsistent with traditional RM models and exhibits longer than anticipated continuum lags

4. Fluctuations in gas density driven by MRI turbulence and convection produce intrinsic variability on the disk’s thermal timescales

5. Long lags between different radii light curves can be measured using cross-correlation and Fourier methods, suggesting an inflow speed on the order of 107−108 cm s−1

6. The differences between the light curves emitted from the thin and thick sides of our simulated disks can be used to estimate disk thickness using RM and PSDs

This research uses 3D multi-frequency radiation magnetohydrodynamic simulations to investigate how AGN disk structure impacts variability and reverberation mapping, revealing that the structure of the disk itself, rather than just the broad line region, is a key factor in explaining observed AGN light curve discrepancies.

1. Improved AI system can predict AGN disk thickness using UV-optical light curves by analyzing the features of emitted light curves as described in Section 5.3, which allows testing the standard thin disk model and estimating accretion rates, noting that the accretion rate for the thick bottom of the disk is nearly double the accretion rate for the thin top of the disk at the inner boundary.

2. Improved AI system can distinguish between intrinsic variability sources by analyzing frequency-binned Power Spectral Densities (PSDs) to determine if variability is driven by MRI turbulence and convection versus X-ray reprocessing, as evidenced by the PSDs of light curves emitted from the top of our simulated disks are very similar to the X-ray PSD, but the PSDs of light curves emitted from the bottom of the disk are different from each other and the X-ray PSD.

3. Improved AI system can identify physical mechanisms causing long lags by analyzing cross-correlation functions using both time-delay methods and Fourier analysis to detect signals consistent with a propagation speed of around 107–108 cm s−1, which are consistent with inflow speeds measured in the simulations.

4. Improved AI system can model AGN disk structure stochastically by recognizing that the asymmetry between the top and bottom disks is present in all four of our simulations because it is a feature of the initial gray simulation from Jiang et al. (2025) and that both disk structures are possible for these AGN parameters but arise stochastically.

5. Improved AI system can assess observational biases by detecting how lag measurements are affected by confounding signals, noting that "the long negative lag we measure in Figure 21 leads to an under-estimate of the light-crossing timescale for the top of the disk and the long positive lag we measure in Figure 20 leads to an over-estimate of the light-crossing timescale for the bottom of the disk."

Variability is a crucial probe of active galactic nuclei (AGN) disks, but the impact of the structure and internal physics of the disk on this variability is poorly understood. We perform 3D multi-frequency radiation magnetohydrodynamic simulations of the 1300-5940 Å emitting region of an AGN disk. This turbulent disk simulation happens to stochastically have different disk thickness above and below the midplane. Light curves emitted from the thin top of the disk are broadly consistent with traditional variability models, where the X-ray irradiation emitted from the corona is the main driver of variability in UV-optical light curves. Conversely, the UV-optical light curves emitted from the thicker bottom of the disk are not strongly correlated with the X-ray light curve and have disk continuum reverberation mapping lags that are 3-5 times longer than the light travel time. While these features are inconsistent with traditional variability models, they are in good agreement with numerous recent observations. Our simulations allow us to interpret these recent observations with a new model for AGN disks, where the thicker disk leads to greater X-ray absorption, less X-ray reprocessing at larger disk radii, and a stronger signal from intrinsic variability driven by fluctuations in the AGN disk. In our simulations, we can measure the inflow timescale of these fluctuations as a long lag between light curves emitted from different disk radii. Understanding how disk structure impacts AGN variability and reverberation mapping can allow us to predict the thickness of AGN disks using UV-optical AGN light curves.