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Abstract: Long Gamma Ray Bursts hold the promise of probing star-formation and metalenrichment up to very high redshifts. The present GRB samples with redshiftdetermination are largely incomplete and therefore a careful analysis ofselection effects is mandatory before any conclusion can be drawn from theobserved GRB redshift distribution. We study and compare three well definedsamples of long GRBs detected by Swift, HETE2 and BeppoSAX. We find that SwiftGRBs are slighly fainter and harder than BeppoSAX and HETE2 GRBs, as expecteddue to the higher energy range in which Swift GRBs are detected and localized,compared to BeppoSAX and HETE2. Gas and dust obscuration plays a role inshaping the GRB samples and the present samples of GRBs with redshift. We arguethat the majority of the bright Swift GRBs without redshift might actually bez<~2 events therefore the present Swift GRB sample with redshift is biasedagainst low-z GRBs. On the other hand, the detection of bright UV rest-frameafterglows from high-z GRBs, and even from those with large X-ray obscuration,implies a dust amount lower than in nearby GRBs,and-or a different dustcomposition. If this is the case, the Swift sample of GRBs with redshifts isprobably a fair sample of the real high-z GRB population. The absence of high-zGRBs in the BeppoSAX and HETE2 samples of GRBs with redshifts is probably dueto the fact at the time of BeppoSAX and HETE2 follow-up faint afterglows ofhigh redshift GRBs will have weaken below the spectroscopic capabilities ofeven 10m class telescopes. The redshift distribution of a subsample of SwiftGRBs with distributions of peak-fluxes, X-ray obscuration and optical magnitudeat a fixed observing time similar to those of the BeppoSAX and HETE2 samples,is roughly consistent with BeppoSAX+HETE2 redshift distribution.

Autor: F. Fiore, D. Guetta, S. Piranomonte, V. D'Elia, L.A. Antonelli


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