Compton tails in long-duration gamma-ray bursts

Some of the phenomena related to the prompt gamma-ray burst emission could be interpreted as the presence of a consistent amount of material (Thomson optical depth similar to1) at rest and close to the source at a typical distance of similar to10(15) cm. The material optical depth tau=sigma <n> DeltaR, where <n> is the average particle density over the distance DeltaR, will attenuate the prompt gamma-ray flux by an amount e(-r), while producing a secondary flux proportional to 1-e(-r) of scattered photons with a partly modified energy spectrum. This emission should be delayed with respect to the prompt gamma-ray emission by a time-scale which is directly related to the burst opening angle (theta(j)) and to the scattering material space distribution (DeltaR).