The BeppoSAX last score in Gamma-Ray Burst astrophysics: the E p,i - E iso relationship

The Italian–Dutch X–ray mission BeppoSAX had a great impact in the field of Gamma-Ray Burst (GRB) astrophysics, in particular with the the discovery of afterglow emission, leading ultimately to the determination of GRB distance scale and energetics, the discovery of new classes of events like dark GRBs, X–ray rich GRBs and X–Ray Flashes (XRF), the first direct evidence of the GRB/SN connection. Here we review the observations and implications of the relationship between the photon energy, Ep,i, at which the intrinsic (i.e. corrected for cosmological redshift) EF(E) spectrum peaks and the isotropic equivalent radiated energy, Eiso, discovered by Amati et al.(2002) based on BeppoSAX measurements when the mission was close to the end of its operation. This relation, subsequently confirmed and extended to the X–ray richest GRBs by HETE–2 measurements, can be used to discriminate among the various scenarios for the prompt emission of GRBs, is a challenging test for jet models and the understanding of the GRB/SN connection and can be also used to build up redshift estimators.