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 where powerful high-frequency emission may be generated: (1) the thermal photosphere, (2) the region of the internal wind instability, and (3) the region of the wind interaction with an ambient gas. This results in a multicomponent structure of GRBs. The emission from the thermal photosphere may be observed as a weak precursor to the main burst. The precursor should have a blackbody-like spectrum with the mean energy of photons of ~1 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 γ-ray missions like the Gamma-Ray Large-Area Space Telescope can clarify the nature of bursters.