GRB 050904: the oldest cosmic explosion ever observed in the Universe

Swift discovered the high redshift (z=6.29) GRB 050904 with the Burst Alert Telescope and began observing with its narrow field instruments only 161 s after the burst onset. GRB 050904 was a long, multi‐peaked, bright GRB with a presence of flaring activity lasting up to 1–2 hours after the burst onset. The spectral energy distribution shows a clear softening trend along the burst evolution with a photon index decreasing from −1.2 up to −1.9. The observed variability is more dramatic than the typical Swift afterglow, the amplitude and rise/fall times of the flares are consistent with the behavior of nearby (z ⩽ 1) long GRBs and suggest the interpretation of the BAT and XRT data as a single continuous observation of long lasting prompt emission.