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Active galactic nuclei (AGNs) have long been observed to "twinkle" (i.e.,their brightness varies with time) on timescales from days to years in theUV/optical bands. Such AGN UV/optical variability is essential for probing thephysics of supermassive black holes (SMBHs), the accretion disk, and thebroad-line region. Here we show that the temperature fluctuations of an AGNaccretion disk, which is magnetically coupled with the corona, can account forobserved high-quality AGN optical light curves. We calculate the temperaturefluctuations by considering the gas physics of the accreted matter near theSMBH. We find that the resulting simulated AGN UV/optical light curves sharethe same statistical properties as the observed ones as long as thedimensionless viscosity parameter $\alpha$, which is widely believed to becontrolled by magnetohydrodynamic (MHD) turbulence in the accretion disk, isabout $0.01$---$0.2$. Moreover, our model can simultaneously explain thelarger-than-expected accretion disk sizes and the dependence of UV/opticalvariability upon wavelength for NGC 5548. Our model also has the potential toexplain some other observational facts of AGN UV/optical variability, includingthe timescale-dependent bluer-when-brighter color variability and thedependence of UV/optical variability on AGN luminosity and black hole mass. Ourresults also demonstrate a promising way to infer the black-hole mass, theaccretion rate, and the radiative efficiency, thereby facilitatingunderstanding of the gas physics and MHD turbulence near the SMBH and itscosmic mass growth history by fitting the AGN UV/optical light curves in theera of time-domain astronomy.

Corona-Heated Accretion-disk Reprocessing (CHAR): A Physical Model to Decipher the Melody of AGN UV/optical Twinkling