“Late Prompt” Emission in Gamma-Ray Bursts?

The flat decay phase in the first 102-104 s of the X-ray light curve of gamma-ray bursts (GRBs) has not yet 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 Γ. 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 Γ of the late shells is larger than 1/θj, where θj is the jet opening angle, we see only a portion of the emitting surface. Eventually, Γ becomes smaller than 1/θj, and the entire emitting surface is visible. Thus, there is a break in the light curve when Γ = 1/θj, 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.