Evidences for two Gamma-Ray Burst afterglow emission regimes

We applied cosmological and absorption corrections to the X-ray and optical afterglow fluxes of a sample of Gamma-Ray Burst sources of known distance. We find a good correlation in X-rays and that the GRBs in our sample form two well defined classes. We tentatively interpret them as radiative and adiabatic afterglow behaviours in the framework of the fireball model for GRBs. We do not observe this correlation at optical wavelengths. This discrepancy with the model may be due to the absorption in the source vicinity.