GRB 050904 at redshift 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 by 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 lightcurve at $t_{\rm b} = 2.6 \pm 1.0$ d. If we assume this is the jet break we derive $E_{\rm jet} \gsim 5 \times 10^{51}$ erg. Our limits show 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.