the value changing between 19.5 and 20.0 years for the bin width at a range of 100-600 days. Throughout the calculations, we use 300 days as the default bin width. In the left panel of Figure \[fig\_period\], we show the binned light curve and the best sinusoidal fit. The original, unbinned light curve is also superposed for the sake of illustration.

    [We note that the LS periodogram is mathematically equivalent to a sinusoidal fit (e.g., @Scargle1983 [@VanderPlas2018]). Here we apply the sinusoidal fit to binned light curve data, for the purpose of alleviating the influences of the highly irregular sampling rate and strong fluctuations on short time scales. By contrast, we calculate the LS periodogram using the unbinned light curve data.]{}

In a nutshell, the above three methods generally [yield]{} periods of $\sim20$ years. We use the period $20.5$ years as the fiducial period in the [analysis that follows]{}.

![image](fig_psd_model.pdf){width="90.00000%"}

Comparison between Periodic and Aperiodic PSD Models
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In Section \[sec\_var\], we apply three aperiodic PSD models to fit the light curve of Ark 120. We also construct periodic models by including a periodic component into the above three PSD models (@Li2018c [@Li2019]). The periodic PSD models have a composite 