Phase shifts measured from the observation (b) by using a different range of phases than adopted for the analysis in Fig. 4 to compute the shifts (Δφ) and a different energy range of the reference pulse profile (ΔE). Specific values of these parameters are indicated in different panels. The shaded intervals represent the centroid energy of the asymmetric Gaussians used to describe the lags with the 1σ uncertainty. The solid line is the best-fit function of the phase lags.