From left to right: power spectra of the Stokes I, Q, and U pure TOIs (black) after applying the lock-in procedure to the raw data in Fig. 4. A bandpass filter ([0.01,2.9] Hz) was applied to reject any high-frequency noise and half wave plate systematic signal (HWPSS) residual at the fourth harmonics. In blue we also show the power spectra after applying the decorrelation procedure to the demodulated data. See Sect. 4.4 for details.