Density clump: forward and long-lived reverse shock models. Left panel: initial distribution of the Lorentz factor (lower part) and kinetic power (upper part) in the flow as a function of injection time t_inj. Middle and right panels: synthetic light-curves at 2 eV (black) and 10 keV (grey) for the forward and reverse shocks, using either the simple one-zone (dotted lines) or the detailed multi-zone (dashed lines) model. The kinetic energies in the spike and extended emission components are  erg and . The adopted microphysics parameters are (forward shock, middle panel) and (reverse shock, right panel). See text for the prescription adopted for the density clump.