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A complete analysis of GRB060607A within the fireshell model: prompt emission, X-ray flares and late afterglow phase

GRB060607A is a very distant ($z=3.082$) and energetic event ($E_{iso}\sim 10^{53}$ erg). Its main peculiarity is that the peak of the near-infrared (NIR) afterglow has been observed with the REM robotic telescope and, interpreting it as the afterglow onset as predicted by the fireball forward shock model, it is possible to infer the initial Lorentz gamma factor of the emitting system. We analyze GRB060607A within the fireshell model. We deal only with the Swift BAT and XRT observations, that are the basic contribution to the GRB emission. We show that the observed temporal variability of the prompt emission can be produced by the interaction of the fireshell with overdense CircumBurst Medium (CBM) clumps. The flares observed in the decaying phase of the X-ray afterglow are also produced by the interaction of the fireshell with CBM clumps, but in a region in which the typical dimensions of the clumps are smaller than the visible area of the fireshell and the peak energy lies in the X-ray band due to the hard-to-soft evolution. We show that it is possible to obtain flares with $\delta t/t_{tot}$ compatible with the observations accounting for the real structure of the CBM clumps. As pointed out by the observations, the remaining part of the X-ray light curve and the NIR emission probably have a different origin than the prompt emission. In particular, the X-ray plateau advocated the presence of a late "energy inection" either from a late activity of the central engine or from the collision with slower material. This new component with respect to the "main" afterglow emission within the fireshell model has been interpreted as the result of a collision between the front layer of the fireshell and a slower tail of matter. We investigated the kinematic of the collision in some specific GRBs that we already analysed within the fireshell model and we found a correlation between the expected Lorentz factor of the slower shell and the Lorentz factor of the fireshell at the transparency, calculated by the numerical modeling of the fireshell dynamics. This correlation reveals clues on the dynamics of the collision and on the matter distribution inside the fireshell at the transparency.

Collisions in the slowing down phase of the promt emission

The fireshell model provides an interpretation of the GRB structure as the joined contribution of the Proper-GRB (P-GRB) and of the afterglow emission, which consists of a rising part, a peak and a decaying phase. One of the purposes of the Swift mission is to observe the first few hours of the afterglow evolution, a largely unexplored window, and indeed it revealed a more complex behavior than expected. In particular the presence in most cases of a plateau advocated the presence of a late "energy inection" either from a late activity of the central engine or from the collision with slower material. The presence of this new component with respect to the "main" afterglow emission within the fireshell model has been interpreted as the result of a collision between the front layer of the fireshell and a slower tail of matter. The identification of this new component motivated a more quantitative study of its origin and characteristics. We investigated the kinematic of the collision in some specific GRBs that we already analysed within the fireshell model and we found a correlation between the expected Lorentz factor of the slower shell and the Lorentz factor of the fireshell at the transparency, calculated by the numerical modeling of the fireshell dynamics. This correlation reveals clues on the dynamics of the collision and on the matter distribution inside the fireshell at the transparency.

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