Cumulative distribution of the afterglow onset time t_p (red solid line) in the observer frame for the 66 long GRBs of the “Gold+Silver” sample. The black dashed line (marked with leftward arrows) is the cumulative distribution of 85 upper limits on t_p (filtered from Table A.1 according to s). The black solid line (shaded yellow region) is the reconstructed t_p distribution (95% confidence interval) accounting for through the Kaplan Meier estimator. The dotted and dot–dashed lines are the cumulative distributions of T₉₀ and T_p,γ (i.e. the time of the peak of the prompt emission light curve), respectively, for the 151 long GRBs. The distributions have been normalised to the respective sample size. Insert: Distribution of t_p (solid blue line) and of upper limits (dashed cyan line with leftward arrows) of the 50 GRBs of our sample (16 with t_p and 34 with ) also present in the Swift BAT complete sample (Salvaterra et al. 2012).