Using GRBÂ 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 ^ { 5 } c .
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 .