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A new spectral energy distribution of photons in the Fireshell Model of GRBs: preliminary results

The behaviour of the N(E) spectrum of the prompt emission of Gamma Ray Bursts (GRBs) is usually described as a ``power-law''. The Fireshell model postulates that the emission process is thermal in the comoving frame of the fireshell and the effective spectrum has a non thermal-shape, being the convolution of various thermal spectra having different temperatures over the Equitemporal Surfaces (EQTS) and observational time. This assumption has been successfully tested on many GRBs; nevertheless, it can be considered just a first approximation. \\ We investigate the possibility of reproducing the observed spectra and light curves of GRBs within the Fireshell Model by assuming a phenomenologically modified black body spectrum for photons changing the energy slope of the spectrum and introducing the index $\alpha$. We obtain all the equations involved in the determination of the GRB luminosity, taking into account the convolution over the EQTS. We test the new model on several GRBs, obtaining in all cases $\alpha \sim -1.8$. We present as specific examples the cases of GRB 080319 and GRB 050904.

GRB 050904: the study of a peculiar high redshift GRB within the Fireshell Model

GRB 050904 is one of the most distant Gamma Ray Bursts ever observed, with a redshift $z=6.29$. This GRB is very peculiar: it presents a strong variability during its entire evolution and the XRT light curve is characterized by a long flaring activity whose characteristics are different from the typical ones observed by Swift XRT. We fit the BAT and XRT data (light curves and spectra) within the Fireshell Model, assuming a modified black body spectrum of photons in the comoving frame of the fireshell. Particular attention is given to the plateau phase: we point out how this component, observed in the X-ray band, shows strong analogies with the usual prompt gamma-ray emission; in particular, the observation of temporal variability instead of a smooth plateau reveals similarities with the prompt emission of GRB 080319B. We consider this component as produced by the injection of slower material into the fireshell.

Analysis of GRB 080319B within the Fireshell Model

GRB 080319B is a very peculiar burst: it is the most luminous GRB observed in the optical energy band and one of the brightest in gamma and X. Furthermore, there is clear evidence that the prompt emission can be separated into two main episodes, partitioned at about 28 s. We analysed the prompt emission light curve and spectrum of this burst within the Fireshell Model. According to the canonical GRB scenario, we identify the first 8 s of emission with the Proper-GRB (P-GRB) and the remaining part of the first episode with the peak of the afterglow, whose variability is produced by the interaction with the Circumburst Medium (CBM). We apply in our analysis a ``modified'' black body spectrum in the comoving frame. We propose that the remaining part of the prompt emission arises from the injection of slower material into the fireshell.

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