Spectrum – energy correlations in Gamma – Ray Bursts confront extremely energetic Fermi GRBs

The origin, reliability and dispersion of the E p,i – E iso and other " spectrum–energy " correlations is a highly debated topic in GRB astrophysics. GRB 080916C, with its huge radiated energy (E iso ∼ 10 55 erg in the 1 keV – 10 GeV cosmological rest–frame energy band) and its intense GeV emission measured by Fermi, gives us a unique opportunity to further investigate this issue. We also include in our analysis another extremely energetic event, GRB 090323, more recently detected and localized by Fermi/LAT and showing a radiated energy comparable to that of GRB 080916C in the 1 keV – 10 MeV energy range. Based on Konus/WIND and Fermi spectral measurements, we find that both events are fully consistent with the E p,i – E iso correlation (updated to 95 GRBs with the data available as of April 2009), thus further confirming and extending it and pointing against a possible flattening or increased dispersion at very high energies. This also suggests that the physics behind the emission of peculiarly bright and hard GRBs is the same as for medium–bright and soft–weak long events (XRFs), which all follow the correlation. In addition, we find that the normalization of the correlation obtained by considering these two GRBs and the other long ones for which E p,i was measured with high accuracy by the Fermi/GBM are fully consistent with those obtained by other instruments (e.g., BeppoSAX , Swift, Konus/WIND), thus indicating that the correlation is not affected significantly by " data truncation " due to detectors thresholds and limited energy bands. The very recent Fermi/GBM accurate estimate of the peak energy of a very bright and hard short GRB with measured redshift, GRB 090510, provides further and robust evidence that short GRBs do not follow the E p,i – E iso correlation and that the E p,i – E iso plane can be used to discriminate and understand the two classes of events. Prompted by the extension of the spectrum of GRB 080916C up to several GeVs (in the cosmological rest–frame) without any excess or cut–off, we also investigated if the evaluation of E iso in the commonly adopted 1 keV – 10 MeV energy band may bias the E p,i – E iso correlation and/or contribute to its scatter. By computing E iso from 1 keV to 10 GeV, the slope of the correlation becomes slightly flatter, while its …