Broadband injection signatures (X‐ray, optical, and radio) for the injection of a kinetic‐energy–dominated shell. Solid lines are light curves with injection, compared to the dashed line case without injection. (a) A violent injection involving formation of new shocks. All the parameters are the same as adopted in Fig. 4. (b) A mild injection. Injective shell parameters are ﹩E_{0,6}=1\times 10^{51}﹩ ergs and ﹩\gamma _{0,6}=200﹩. The central engine activity time for the rear shell is ﹩\delta T_{6}=20﹩ s; the time interval between ejecting the two shells is ﹩\Delta T_{36}=20﹩ s. Other parameters are the same as in Fig. 4 and in (a). Notice that although the total energy for shell 6 is higher in the present case, the relative Lorentz factor between the two shells is much smaller, and the injection is mild.