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 ∼1) at rest and close to the source at a typical distance of ∼1015 cm. The material optical depth τ=σ〈n〉ΔR, where 〈n〉 is the average particle density over the distance ΔR, will attenuate the prompt gamma‐ray flux by an amount e−τ, while producing a secondary flux proportional to 1 − e−τ 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 (θj) and to the scattering material space distribution (ΔR).