Late prompt emission in gamma-ray bursts?

The flat decay phase in the first 10(2) - 10(4) s of the X-ray light curve of gamma-ray bursts (GRBs) has not yet 2 4 10 - 10 been convincingly explained. The fact that the optical and X-ray light curves are often different, with breaks at different times, makes problematic any explanation based on the same origin for both the X-ray and optical fluxes. We here assume that the central engine can be active for a long time, producing shells of decreasing bulk Lorentz factors G. We also assume that the internal dissipation of these late shells produces a continuous and smooth emission ( power law in time), usually dominant in X-rays and sometimes in the optical. When Gamma of the late shells is larger than 1/theta(j), where theta(j) is the jet opening angle, we see only a portion of the emitting surface. Eventually, Gamma becomes smaller than 1/theta(j), and the entire emitting surface is visible. Thus, there is a break in the light curve when Gamma = 1/theta(j), which we associate with the time at which the plateau ends. After the steeply decaying phase that follows the early prompt, we see the sum of two emission components: the late- prompt emission (due to late internal dissipation), and the real afterglow emission (due to external shocks). A variety of different optical and X-ray light curves are then possible, explaining why the X-ray and the optical light curves often do not track each other ( but sometimes do), and often they do not have simultaneous breaks.