Implications of the consistency of the peculiar GRBs 060218 and 060614 with the E p , i – E iso correlation

We analyze and discuss the location in the E p,i – E iso plane of two peculiar Gamma–Ray Bursts: GRB 060218 and GRB 060614. GRB 060218 is important because of its very low redshift z = 0.033, its low isotropic–equivalent radiated energy E iso ∼ 7 × 10 49 erg and its clear association with a supernova (SN) event (SN 2006aj). Under these respects, GRB 060218 is the event most similar to GRB 980425, the proto–type event of the GRB–SN connection. However, while the peak energy E p,i and E iso values of GRB 980425 are inconsistent with the E p,i – E iso correlation holding for long GRBs/XRFs, GRB 060218 is fully consistent with it. This evidence, combined with the 'chromatic' behavior of the afterglow light curves, is at odds with the hypothesis that it was a 'normal' GRB seen off-axis, one of the popular explanations for the inconsistency of GRB 980425 with the E p,i – E iso correlation, and supports the existence of a class of truly sub–energetic GRBs. GRB 060614 is an event with deep upper limits to the luminosity of an associated SN ruling out the hypothesis that it was originated by a hypernova. Based on published spectral information, we find that also this event is likely consistent with the E p,i – E iso correlation. We also include in our analysis other GRBs with clear evidence of association with a SN and two more events with deep limits to the magnitude of an associated SN. We discuss the implications of our results for the existence and rate of sub–energetic GRBs, the GRB/SN connection and the properties of the newly discovered sub–class of long GRBs not associated with a hypernova.