GRB 050904 at red shift 6.3: Observations of the oldest cosmic explosion after the Big Bang

We present optical and near-infrared observations of the afterglow of the gamma-ray burst GRB 050904. We derive a photometric redshift z = 6.3, 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 $3.4 \times 10^{53}$ 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 $t_{\rm b} = 2.6 \pm 1.0$ d (observer frame). If we assume this is the jet break, we derive a beaming-corrected energy $E_\gamma \sim (4 \div 12) \times 10^{51}$ 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 z > 6 and shows that GRBs are a powerful tool to study the star formation history up to very high redshift.