THE SPECTRUM OF GAMMA-RAY BURST: A CLUE

In the standard baryonic fireball model, the prompt MeV emission of the Gamma-ray bursts has been attributed to the synchrotron radiation of the electrons accelerated in the internal shocks. Such a model, if correct, requires a very high bulk Lorentz factor of the fast shells. The thermal radiation of these fast shells is too strong to be hidden by the non-thermal internal shock emission. The non-detection of such a thermal component in the spectrum of most GRBs in turn challenges the baryonic fireball model.