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 $\tau_\text{d}$ consistent with observations. The scaling relation between $\tau_\text{d}$ and the rest-frame wavelength $\lambda$ has a bended shape, $\tau_\text{d}\propto\lambda$ at short wavelengths and transitioning to a plateau at long wavelengths. At $\lambda=2500\text{\AA}$, 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.