Residual distribution of the optical phase of the main pulse around the best fit shown in Fig. 2. Each point represents the number of phase residuals N_i in the ith bin. The error bar on each bin is . The three datasets refer to all observations (circles), to observation 4 (squares) and observation 1 (diamonds). The bin widths are 1.5   μs, 4.1   μs, and 7.3   μs. The (red) solid lines superimposed to each dataset represent the best-fitting Gaussian with σ ~ 32   μs (circles), σ ~ 24   μs (squares) and σ ~ 54   μs (diamonds).