grid width of $\log(1.3)$ and a minimum of two power measurements per bin (@Uttley2002). The uncertainties of the binned powers are set to be the Poisson noise power.

[@Lobban2018] measured the PSD of Ark 120 in the X-ray band (0.3-10 keV) using $\sim 420$ ks [*XMM- Newton*]{} monitoring data and additional [*Swift*]{}, [*RXTE*]{}, and [*NuSTAR*]{} observation data (see their Figure 13). We adjust the normalization of the X-ray PSD to align with the optical PSD and superpose it upon the optical PSD in Figure \[fig\_psd\]. Figure \[fig\_psd\] illustrates that both the optical and X-ray PSDs can be universally described by a single power law with a slope of $\sim -1.6$, [despite some weak features in the]{} $f\times P(f)$ plot. The similar shapes of the optical and X-ray PSDs potentially imply that the optical and X-ray variabilities may have the same driving mechanism. A plausible scenario is reprocessing of X-ray emission into the UV/optical band (@Guilbert1988).

We apply three widely used PSD models to fit the optical light curve and compare the relative merits of these three models using the framework [`RECON`]{} developed in [@Li2018c]. This framework parameterizes [the]{} AGN time series in [the]{} frequency domain and transforms it back to 