Spectrum-Energy Correlations and Swift GRBs

Abstract

The correlation between the photon energy at which the redshift corrected nFn spectrum peaks (hence called "peak energy", Ep,i) and the isotropic equivalent radiated energy (Eiso), is one of the most intriguing and debated observational evidences in Gamma- Ray Bursts (GRB) astrophysics. In order to fully exploit this correlation to understand GRB physics and use them as a cosmological tool, we need to understand and reduce selection effects and biases in the sample of GRBs with known redshift. The Swift satellite, thanks to its scientific payload and unprecedented slewing capabilities, is providing us new observational evidences and a reduction of selection effects in the process of GRB redshift estimate. I discuss the location of Swift GRBs in the Ep,i-Eiso plane and the main implications, with particular emphasis on selection effects, the existence and nature of sub-energetic long GRBs, the GRB-SN connection, the difference between long and short GRBs and the recently discovered sub-class of long GRBs without association with a hypernova. I also briefly review the impact of Swift observations on 3-parameters spectrum-energy correlations, which have been proposed as a potentially powerful tool to standardize GRBs for the estimate of cosmological parameters.