Prompt and delayed emission properties of gamma-ray bursts observed with BeppoSAX

We investigated the spectral evolution in the 2-700 keV energy band of gamm a-ray bursts (GRBs) detected by the Gamma-Ray Burst Monitor (GRBM) and loca lized with the Wide Field Cameras (WFCs) aboard the BeppoSAX satellite befo re 1998 May. Most of them have been followed up with the Narrow Field Instr uments aboard the same satellite. In the light of these results we discuss open issues on the GRB phenomenon. We find that the optically thin synchrot ron shock model (SSM) provides an acceptable representation of most of the time-resolved GRB spectra extending down to 2 keV, except in the initial ph ases of several bursts and during the whole duration of the quite strong GR B 970111, where a low-energy photon depletion with respect to the thin SSM spectrum is observed. A strong and time-variable low-energy cutoff, consist ent with absorption effect, is observed during the prompt emission of GRB 9 80329. We find that the X-ray afterglow starts at about 50% of the GRB dura tion and that its fluence, as computed from the WFC light curve, is consist ent with the decay law found from the afterglow NFI observations. We also i nvestigate the hydrodynamical evolution of the GRB in our sample and their associated afterglow, when it was detected. We find that the photon index o f the latest spectrum of the GRB prompt emission is correlated with the ind ex of the afterglow fading law, when available, as expected on the basis of an external shock of a relativistic fireball. We also find that for most o f the GRBs in our sample the late emission is consistent with a slow coolin g of the shock. Adiabatic shocks appear more likely than radiative shocks. Parameters of the shocks at earliest times have been derived.