GRB960720: very fast spectral evolution

As can be inferred also from the X and gamma light curves, this event, in front of the relatively short duration ($\sim$8s in gamma and $\sim$17s in X) presents a very strong and fast spectral evolution. The lack of X-ray emission in the first s indicates some absorption mechanism operating during the very beginning of the event. Due to the low statistical quality of the data, broad band spectral analysis of GRB960720 ([Piro et al. 1998a]) has been performed by assuming a fast evolving synchrotron spectrum characterized by self-absorption at a critical energy E_c and a break energy E₀ corresponding to the minimum energy of the electron distribution. Below E_c we assumed the optically thick slope of 1.5; between E_c and E₀ we have adopted the slope of -0.67, expected by optically thin emission below the minimum energy of the electron distribution. Finally, we assumed for the emission determined by the electron distribution a power-law spectral index of -2.4, corresponding to the asympotical value obsered in the last part of the event.
The fits were performed by using the Band form, with alpha, beta and E₀ corresponding to 1.5, E_c and -0.67 in the first two time intervals, and to -0.67, E₀ and -2.4 in the subsequent intervals.
The results, reported in Tab. 5.2, indicate a very fast spectral evolution, with E_c evolving from $\sim$ 520 keV to $\sim$6 keV in 1s. Also the evolution of E₀ is extrimely fast. These results challange some fireball models in which self-absorption should be observed only at much lower energies ([Metzger et al. 1997]).