On the structures in the afterglow peak emission of gamma ray bursts
  Using GRB 991216 as a prototype, it is shown 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 (ABM) pulse with inhomogeneities in the interstellar medium (ISM). The initial phase of such process occurs at a Lorentz factor $\gamma\sim 310$. The crossing of ISM inhomogeneities of sizes $\Delta R\sim 10^{15}$ cm occurs in a detector arrival time interval of $\sim 0.4$ s implying an apparent superluminal behavior of $\sim 10^5c$. The long lasting debate between the validity of the external shock model vs. the internal shock model for GRBs is solved in favor of the first. 
R. Ruffini, C.L. Bianco, P. Chardonnet, F. Fraschetti, S.-S. Xue