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About 15% of Gamma Ray Bursts have precursors, i.e. emission episodespreceding the main event, whose spectral and temporal properties are similar tothe main emission. We propose that precursors have their own fireball,producing afterglow emission due to the dissipation of the kinetic energy viaexternal shock. In the time lapse between the precursor and the main event, weassume that the central engine is not completely turned off, but it continuesto eject relativistic material at a smaller rate, whose emission is below thebackground level. The precursor fireball generates a first afterglow by theinteraction with the external circumburst medium. Matter injected by thecentral engine during the "quasi-quiescent" phase replenishes the externalmedium with material in relativistic motion. The fireball corresponding to themain prompt emission episode crashes with this moving material, producing asecond afterglow, and finally catches up and merges with the first precursorfireball. We apply this new model to GRB 091024, an event with a precursor inthe prompt light curve and two well defined bumps in the optical afterglow,obtaining an excellent agreement with the existing data.

Afterglows from precursors in Gamma Ray Bursts. Application to the optical afterglow of GRB 091024