GRB970111: high statistics

GRB970111 spectral evolution shows a peculiar behavior not clearly observed in the other GRBs in our sample: its photon index $\Gamma_{X}$ does not change with time, while $\Gamma_{\gamma}$ becomes softer and softer. Our energy passband during the primary events, while the $\Gamma_{\gamma}$ variation does not appear to be consistent with the expected change in the case of a synchrotron shock that propagates in a medium of uniform density with constant electron energy distribution. It appears more consistent with a softening of the electron distribution slope with time. If this interpretation is correct, this could account of the non detection or, at most, detection at a very low flux level of X-ray afterglow emission ($1.2\pm0.3 \times 10^{-13}\, erg cm^{-2} s^{-1}$ after 17 hrs from the GRB onset ([Feroci et al. 1998]). Indeed in this case the spectral index p (= -2($\Gamma_{\gamma}$+1) or = -2$\Gamma$-1, [Sari, Piran and Narayan 1998]) could become higher than 3. As a consequence, if no electron re-acceleration occurs, we would expect an index of the afterglow power law decay ($\propto t^{-\delta}$), that in the case the peak energy is below our energy passband is given ([Sari, Piran and Narayan 1998]) by $\delta \,=\, (3p-2)/2$for adiabatic cooling and $\delta \,=\,(6p-2)/2$ for radiative cooling (in the case E_p is above our energy passband is given by $\delta \,=\,3(p-1)/4$) ([Wijers et al. 1997,Sari, Piran and Narayan 1998]). In both cases it results $\delta$ at least higher than 1.5, that is consistent with the likely result obtained by Feroci et al. (1998) .