LSP of the 0.3–2 keV light curve during phase 1b. The red solid curve represents the best-fit power-law + constant model. The blue vertical line marks the frequency of the QPO at approximately 1 × 10⁻⁶ Hz (∼12 days). The lower panel shows the data-to-model ratio as a function of frequency, where the QPO feature is clearly visible after subtracting the continuum modeled by the power-law + constant model.