The Ep,i - Eiso correlation in GRBs: updated observational status, re-analysis and main implications

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  • 12 July 2006
Abstract
The correlation between the cosmological rest-frame nuFnu spectrum peak energy, Ep,i, and the isotropic equivalent radiated energy, Eiso, discovered by Amati et al. in 2002 and confirmed/extended by subsequent osbervations, is one of the most intriguing and debated observational evidences in Gamma-Ray Bursts (GRB) astrophysics. In this paper I provide an update and a re-analysis of the Ep,i - Eiso correlation basing on an updated sample consisting of 43 long GRBs/XRFs with firm estimates of z and Ep,obs, 10 GRBs with uncertain valeus of z and/or Ep,obs, 2 short GRBs with firm estimates of z and Ep,obs,i and the peculiar sub-energetic event GRB980425/SN1998bw. In addition to standard correlation analysis and power-law fitting, the data analysis here reported includes a modelization which accounts for sample variance. All 53 classical long GRBs and XRFs, including 11 Swift events with published spectral parameters and fluences, have Ep,i and Eiso values, or upper/lower limits, consistent with the correlation, which shows a chance probability as low as 7x10^(-15), a confirmed slope of about 0.5 and an extra-Poissonian logarithmic dispersion of about 0.15, it extends over about 5 orders of magnitude in Eiso and about 3 orders of magnitude in Ep,i and holds from the closer to the higher z GRBs. Sub-energetic GRBs (980425 and possibly 031203) and short GRBs are found to be inconsistent with the Ep,i - Eiso correlation, showing that it can be a powerful tool for discriminating different classes of GRBs and understanding their nature and differences. I also discuss the main implications of the updated Ep,i - Eiso correlation for the models of the physics and geometry of GRB emission, its use as a pseudo-redshift estimator and the tests of possible selection effects with GRBs of unknown redshift.

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