MG11 
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The Ep.i - Eiso correlation in Gamma-Ray Bursts

The correlation between the cosmological rest-frame photon frequency at which the nuFnu spectrum peaks (Ep,i) and the total isotropic-equivalent radiated energy (Eiso) is one of the strongest and most intriguing recent observational evidences concenring cosmological Gamma-Ray Bursts (GRBs). I briefly review the observational scenario and the main implications of this correlation, which include the physics of GRB prompt emission, the jet structure and geometry, GRB/XRF/SN unification models, the comprehension of the different origin of short and long GRBs, the nature of peculiar events like sub-energetic GRBs.

The Ep,i-Eiso correlation and the nature of sub-energetic GRBs

GRB 060218 is a peculiar event which shares several properties with GRB980425, the proto-type event of the GRB-SN connection: a very low redshift (0.0331), a very low isotropic-equivalent radiated energy, a clear association with a SN event (SN2006aj). However, while the Ep,i and Eiso values of GRB980425 are completely inconsistent with the Ep,i-Eiso (Amati) correlation holding for long GRBs/XRFs (as is possibly true for the other sub-energetic event GRB031203), those of GRB060218 are fully consistent with it. This evidence, togehter with the "achromatic" behaviour of the GRB060218 afterglow light curve and some properties of SN2006aj, challanges the popular explanations for the nature of sub-energetic GRBs (like, e.g., the very off-axis scenario) and points towards the existence of a (still mostly uncovered) population of intinsecally faint GRBs.