Precursors of gamma-ray bursts: A clue to the burster's nature

In relativistic strongly magnetized winds outflowing from the fast-rotating compact progenitors of gamma-ray bursts (GRBs), there are three regions wh ere powerful high-frequency emission may be generated: (1) the thermal phot osphere, (2) the region of the internal wind instability, and (3) the regio n of the wind interaction with an ambient gas. This results in a multicompo nent structure of GRBs. The emission from the thermal photosphere may be ob served as a weak precursor to the main burst. The precursor should have a b lackbody-like spectrum with the mean energy of photons of similar to1 MeV, and its intensity should be tens to hundreds of times smaller than that of the main GRB emission. Observations of such precursors with future gamma -r ay missions like the Gamma-Ray Large-Area Space Telescope can clarify the n ature of bursters.