Understanding the UV/Optical Variability of AGNs through Quasi-Periodic Large-scale Magnetic Dynamos

Abstract

The physical origin of the recently identified slow-moving temperature fluctuations in accretion disks around super-massive black holes (SMBHs) cannot be accounted for by reverberation models. In this work, we propose that large-scale dynamos (LSDs) operating in accretion disks could generate quasi-periodic perturbations in the turbulence viscosity, thereby producing outward-going temperature fluctuations with speeds comparable to those inferred from observations. Furthermore, we find that the UV/optical fluxes of our model are compatible with a damped-random-walk (DRW) process, with a damping time $τ_\text{d}$ consistent with observations. The scaling relation between $τ_\text{d}$ and the rest-frame wavelength $λ$ has a bended shape, $τ_\text{d}\proptoλ$ at short wavelengths and transitioning to a plateau at long wavelengths. At $λ=2500\textÅ$, the damping time roughly follows $\propto M_\text{BH}^{1/2}$ when $M_\text{BH}\gtrsim 10^6M_\odot$, consistent with observational constraints, though it tends to be underestimated for lower SMBH masses. Including additional refinements, such as the dependence of dynamo properties on $M_\text{BH}$ and AGN luminosity, and accounting for X-ray reprocessing, would further enhance the accuracy of the model. In addition, we show that generic disk models with spatially uncorrelated fluctuations cannot explain the observed DRW damping times; spatially correlated fluctuations, such as those discussed in this paper, may be an essential ingredient.

Figures (10)

• Figure 1: Comparing our prescription Equation (\ref{['eqn:alphass(B)']}) with the global disk dynamo simulation in Zhou2024. Panels (a) and (b) are outputs of run AO1 in Zhou2024. Panel (a) shows the space-time diagram of $\overline B_\phi(t,\theta)$ at $r=2r_\text{g}$, starting from $t_0=10^4\Omega_0^{-1}$, and $\theta$ is the latitude. Panel (b) plots the normalized Maxwell stress at $\theta=0.15$, with $t_0=5960\Omega_0^{-1}$. Panel (c) shows the varying part in Equation (\ref{['eqn:alphass(B)']}) using the parameters $\alpha=0.3$, $\epsilon_0=0.1$, $C_l=30$, and $C_\Omega=20$, starting from $t_0=2\times10^6\Omega_0^{-1}$. The arrows in panels (b) and (c) remark a particular peak in each panel which propagates, damps, and merges with a later peak.
• Figure 2: Relative temperature fluctuations for the fiducial run. The dashed curve indicates the trajectory of a patch moving at the local dynamo wave speed.
• Figure 3: Results for the fiducial run: (a) The time series of accretion rate at the inner boundary. (b) The corresponding PSD. In the right panel, the red lines indicate $\propto f^{-1}$ (left) and $\propto f^{-3}$ (right), respectively. The vertical line indicates the local dynamo frequency, $f=\Omega_0/C_\Omega$.
• Figure 4: Results for the fiducial run. (a) The bolometric luminosity normalized by the Eddington luminosity $L_\text{Edd}$, and the inset is a zoom-in plot for the $t-t_0=800-1000$ days interval on the same vertical scale. (b) The rms-flux correlation for the binned flux. (c) The PDF of the flux and a log-normal fit (red curve).
• Figure 5: DRW-model fitting for the fiducial run. Top panel: The normalized specific flux at $2500\text{\AA}$ binned with a $10$-day interval, and its DRW prediction. Bottom-left panel: Corner plot of the fitted DRW amplitude $\sigma_\text{DRW}$, damping time $\tau_\text{DRW}$, and white-nose term $\sigma_\text{n}$. The vertical dashed lines indicate the $16$th, $50$th, and $84$th percentiles, respectively. Bottom-right panel: The PSD and the fitted DRW model. In the top and bottom-right panels, the orange shaded areas demark the $1\sigma$ uncertainty. In the bottom panels, the red shaded regions mark time scales greater than $20\%$ of the light curve length and less than the mean cadence.