A mean redshift of 2.8 forSwiftgamma-ray bursts

P Jakobsson,J Sollerman,B L Jensen,T Pursimo,A J Castro-Tirado,J M Castro Cerón,S G Sousa,R Priddey,J Gorosabel,J P U Fynbo,Anna Árnadóttir ,Priyamvada Natarajan ,H J Deeg,Kim Steenstrup Pedersen ,Derrick G Watson ,Da Fiuza ,J Hjorth,Cj Davis ,S Mykolaitis ,A J Levan ,N R Tanvir

doi:10.1051/0004-6361:20054287

Abstract

The exceptionally high luminosities of gamma-ray bursts (GRBs), gradually emerging as extremely useful probes of star formation, make them promising tools for exploration of the high-redshift Universe. Here we present a carefully selected sample of Swift GRBs, intended to estimate in an unbiased way the GRB mean redshift (z_mean), constraints on the fraction of high-redshift bursts and an upper limit on the fraction of heavily obscured afterglows. We find that z_mean = 2.8 and that at least 7% of GRBs originate at z > 5. In addition, consistent with pre-Swift observations, at most 20% of afterglows can be heavily obscured. The redshift distribution of the sample is qualitatively consistent with models where the GRB rate is proportional to the star formation rate in the Universe. We also report optical, near-infrared and X-ray observations of the afterglow of GRB 050814, which was seen to exhibit very red optical colours. By modelling its spectral energy distribution we find that z = 5.3 +/- 0.3. The high mean redshift of GRBs and their wide redshift range clearly demonstrates their suitability as efficient probes of galaxies and the intergalactic medium over a significant fraction of the history of the Universe.

Highlights

The potential of long-duration (>2 s) gamma-ray bursts (GRBs) as probes of the high-redshift Universe has long been recognised
The high mean redshift of GRBs and their wide redshift range clearly demonstrates their suitability as efficient probes of galaxies and the intergalactic medium over a significant fraction of the history of the Universe
The GRB population observed until the end of 2004 had enabled much progress in the field, it was widely expected that the launch of Swift, and the subsequent order of magnitude increase in the number of GRBs open to detailed study, would allow further insight into the high-redshift Universe (Gehrels et al 2004)

Summary

Introduction

The potential of long-duration (>2 s) gamma-ray bursts (GRBs) as probes of the high-redshift Universe has long been recognised. With multi-wavelength observations it is possible to distinguish the curved red spectrum expected for an extinguished burst, from the sharp cut-off due to the Lyα break. This method is similar to that which has been used successfully (and accurately) in the selection and study of Lyman-break galaxies (Steidel & Hamilton 1992, 1993), but is likely to be even more robust thanks to the simple power-law spectra exhibited by GRB afterglows.

Observations

Spectral energy distribution of the afterglow

The redshift distribution of Swift bursts

Findings

Discussion and conclusions