When GRB afterglows get softer, hard components come into play

We investigated the ability of simple spectral models to describe the early afterglow emission of GRBs. We found that four spectra, belonging to the GRB060502A, GRB060729, GRB060904B, GRB061H0A prompt-afterglow transition phase, can be modeled neither by a single power-law nor by the Band model. Instead we find that the data present high-energy ( > 3 keV, in the observer frame) statistically significant excesses with respect to these models In all four cases, the deviations can be modeled well by adding either a second power law or a blackbody component to the usual synchrotron power law spectrum. Although the data do not allow an unequivocal physical interpretation, the importance of this analysis consists in showing that a simple power-law model or a Band model is insufficient to describe the X-ray spectra of a small homogeneous sample of GRBs at the end of their prompt phase [1].