Fourier lag-frequency spectra measured in ObsID 0124930301. The measured lag-frequency spectra τ₁ and τ₂ are denoted by the closed and opened symbols, respectively (see text for details). The green squares and inverted triangles are the lag-frequency spectra calculated by the linearly interpolated LCs, while the blue points and triangles are the lag-frequency spectra calculated by the simulated LCs with GP. The orange dotted and dashed lines respectively represent the theoretical time-lag spectra expected from Case A and B in the frame of the classical one-zone synchrotron self-Compton (SSC) model. The horizontal dashed lines denote the time lags approximately calculated by the analytical method for Case A. The orange solid lines are the theoretical predictions from our proposed two-zone SSC model. The detail discussion is presented in Sect. 6.