Evolution of the physical conditions in the shocked region. Left panel: the evolution of the Lorentz factor Γ_∗ (top), the specific energy density ϵ_∗ (middle), and the mass density ρ_∗ (bottom) is plotted for the reference Case B (Case A would show exactly the same evolution), Case B with a sharp distribution of the initial Lorentz factor (see text) and Case B with a constant ejected mass flux Ṁ rather than a constant injected kinetic power Ė (see text). For each case, two curves are seen, corresponding to the propagation of “forward” and “reverse” internal shocks. Middle panel: corresponding spectral evolution (same colour code) assuming a constant fraction ζ of accelerated electrons. Right panel: corresponding spectral evolution (same colour code) assuming a varying fraction ζ of accelerated electrons (see text).