PSDs of light curves in 2003 (right panel) and 2017 (right panel). The red solid lines indicate the fitting results of the power law, and the corresponding spectral indexes α are denoted in the panel. The pink horizontal lines represent the power level of the noise floor. There is an obvious dip (oscillation) feature at the ν ∼ 10^−3.6 Hz of the PSD in 2003, and the red dotted line indicates the fitting result after adding a Gaussian component to be responsible for the dip feature.