GRB050904 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 GRB050904. 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 x 10(53) erg in the rest- frame 110 110- 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(b) = 2.6 +/- 1.0 d ( observer frame). If we assume this is the jet break, we derive a beaming-corrected energy E-gamma similar to (4 divided by 12) x 10(51) erg. This limit shows that GRB050904 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.