On the structures in the afterglow peak emission of gamma-ray bursts

Using GRB 991216 as a prototype, we show that the intensity substructures observed in what is generally called the prompt emission in gamma-ray bursts (GRBs) do originate in the collision between the accelerated baryonic matter pulse with inhomogeneities in the interstellar medium (ISM). The initial phase of such process occurs at a Lorentz factor gamma similar to 310. The crossing of ISM inhomogeneities of sizes occurs in a DeltaR similar to 10(15) cm detector arrival time interval of similar to0.4 s, implying an apparent superluminal behavior of similar to10(5)c. The long-lasting debate between the validity of the external shock model versus the internal shock model for GRBs is solved in favor of the first.