The canonical Gamma-Ray Bursts and their "precursors"

Abstract. The fireshell model for Gamma-Ray Bursts (GRBs) naturally leads to a canonical GRB composed of a proper-GRB (P-GRB) and an afterglow. P-GRBs, introduced by us in 2001, are sometimes considered “precursors ” of the main GRB event in the current literature. We show in this paper how the fireshell model leads to the understanding of the structure of GRBs, with precise estimates of the time sequence and intensities of the P-GRB and the of the afterglow. It leads as well to a natural classification of the canonical GRBs which overcomes the traditional one in short and long GRBs. Keywords: Gamma-Ray: Bursts PACS: 98.70.Rz The so-called “prompt emission ” light curves of many Gamma-Ray Bursts (GRBs) present a small pulse preceding the main GRB event, with a lower peak flux and separated by this last one by a quiescent time. The nature of such GRB “precursors ” is one of the most debated issues in the current literature [see Ref. 1, as well as G. Ghisellini’s talk at this Meeting]. Already in 2001 [2], within the “fireshell ” model, we proposed that GRB “precursors ” are the Proper GRBs (P-GRBs) emitted when the fireshell becomes transparent, and we gave precise estimates of the time sequence and intensities of the P-GRB and the of the afterglow. Within our approach, in fact, we assume that all GRBs originate from the gravitational collapse to a black hole [2, 3]. The e ± plasma created in the process of the black hole formation reaches thermal equilibrium on a time scale on