Reference pulse profile of 4U 0115 + 63 extracted from observation (b). Data are from the PCA (energy range 8-10.5 keV). The pulse profile has been linearly scaled to have zero average and unitary standard deviation. The two vertical solid lines enclose the phase interval used for the correlation analysis: 0.33–0.72 (the phase is shifted with respect to Fig. 3 for clarity’s sake).