Title:  On the Structures in the Afterglow Peak Emission of 
        Gamma-Ray Bursts


Remo Ruffini
Universita' La Sapienza - Roma


Abstract:
 
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 ~310. The crossing 
of ISM inhomogeneities of sizes R~10^15 cm occurs in a detector arrival time 
interval of ~0.4 s, implying an apparent superluminal behavior of ~10^5c. 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.