Thanks to Swift, rapid follow-up studies of GRBs are possible. About 1/3 of all GRBs observed by Swift has measured redshift (an enormous improvement with respect to the pre-Swift era). Salvaterra et al recently defined a subsample of Swift GRBs with 90% of redshift completeness, based on favorable observing conditions (Jakobsonn et al) and prompt brightness. We present their spectral properties and for the first time investigate the spectral-energy correlations in a sample unaffected by biases induced by the measurement of z. Using the complete sample as reference, we simulate samples of GRBs to probe the role of the flux threshold on the Epeak − L relation. We found that the use of flux limited samples in correlation studies cannot be responsible for the existence of the correlation itself.
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