GRB 050904 at redshift 6.3: observations of the oldest cosmic explosion after the Big Bang

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Abstract
We present optical and near-infrared observations of the afterglow of the gamma-ray burst GRB 050904. We derive a photometric redshift , estimated from the presence of the Lyman break falling between the I and J filters. This is by far the most distant GRB known to date. Its isotropic-equivalent energy is erg in the rest-frame 110-1100 keV energy band. Despite the high redshift, both the prompt and the afterglow emission are not peculiar with respect to other GRBs. We find a break in the J-band light curve at d (observer frame). If we assume this is the jet break, we derive a beaming-corrected energy erg. This limit shows that GRB 050904 is consistent with the Amati and Ghirlanda relations. This detection is consistent with the expected number of GRBs at and shows that GRBs are a powerful tool to study the star formation history up to very high redshift.