Upper limits on the initial bulk Lorentz factor (as a function of the redshift) derived under the assumption of a leptonic external shock origin of the high-energy emission. Red triangles mark GRBs with measured redshift. For the other events a redshift ranging between 0.01 and 10 is assumed. Higher flux GRBs are marked with lighter curves. The orange region shows the Γ₀ distribution derived by Ghirlanda et al. (2012) for a sample of GRBs with afterglow peak time measured from optical data.