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 γ ~ 310. The crossing of ISM inhomogeneities of sizes ΔR ~ 1015 cm occurs in a detector arrival time interval of ~0.4 s, implying an apparent superluminal behavior of ~105c. 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.