Left: Lorentz factor Γ_γγ for the time intervals c (red), d₁ (blue), and d₂ (green) as a function of the ratio of the emission radii of the GeV and MeV photons, assuming that the high-energy spectral break comes from photon opacity to pair creation (Eq. (8)). The dashed lines represent the lower limit of the Lorentz factor for transparency, Γ_tr (Eq. (13)). The shaded strips indicate the typical uncertainty on these quantities, obtained by propagating the errors on the measured values listed in Table 3. Right: MeV (full lines) and GeV (dashed lines) emission radii as a function of the Lorentz factor. The dotted lines correspond to the photospheric radius R_ph in the different time intervals. The deceleration radius is not plotted, but we checked that it is always well above R_ph, R_MeV, and R_GeV for normal densities in the external medium (assuming either a wind or a uniform medium).