Emission processes in gamma-ray bursts

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Recent results of the hectic research activity about gamma-ray bursts will be reviewed, with emphasis about the emission processes at the origin of the observed gamma-rays. The conventional synchrotron shock scenario is found to have problems, due to the very short cooling times of the emitting electrons, which implies a predicted spectrum F(nu) proportional to nu(-1/2), much steeper than what is observed. It is therefore compelling to look for alternative emission processes, such as quasi-thermal Comptonization, implying the presence of mildly or sub-relativistic electrons, producing, through multiple Compton scatterings, a spectrum proportional to nu⁰ ending with a Wien peak where photons and electron energies balance. The afterglow light, instead, can be indeed due to synchrotron radiation, and a confirmation of this is the recently detected optical linear polarization of the afterglow of GRB 990510. Some consequences of this discovery will be outlined. A quantum leap in our understanding of the physics of gamma-ray bursts is expected to come with SWIFT, a NASA-MIDEX dedicated satellite.