An intrinsic smoothing mechanism for gamma-ray burst spectra in the fireball model

It is shown here that the differential Doppler shift of different patches o f the blast-wave front in the fireball model at varying angles to the line of sight could provide an intrinsic smoothing mechanism for the spectra of gamma-ray bursts (GRBs) and the associated afterglows at lower energy bands . For the model parameters of interest, a blast-wave comoving frame monochr omatic spectrum at nu is smoothed to have a half-width at half-maximum of s imilar to(0.6-2.0)nu. Some other implications of this smoothing process are discussed. In particular, if the circumburster medium is uniform and elect ron and magnetic energies are fixed fractions of the total postshock energy with time, the observed GRB or afterglow spectra cannot be steeper than nu (alpha) with alpha = 0.75-1.25, regardless of the intrinsic spectra in the comoving shock frame; i.e., a very steep electron distribution function (wi th p > 3) could produce an observed spectrum with alpha similar to 1. In ad dition, a generic fast-rise-slow-decay type of GRB temporal profile is expe cted.