Gamma-ray burst ( GRB ) 050904 is the most distant X-ray source known , at z = 6.295 , comparable to the farthest AGN and galaxies .
Its X-ray flux decays , but not as a power-law ; it is dominated by large variability from a few minutes to at least half a day .
The spectra soften from a power-law with photon index \Gamma = 1.2 to 1.9 , and are well-fit by an absorbed power-law with possible evidence of large intrinsic absorption .
There is no evidence for discrete features , in spite of the high signal-to-noise ratio .
In the days after the burst , GRB 050904 was by far the brightest known X-ray source at z > 4 .
In the first minutes after the burst , the flux was > 10 ^ { -9 } erg cm ^ { -2 } s ^ { -1 } in the 0.2–10 keV band , corresponding to an apparent luminosity > 10 ^ { 5 } times larger than the brightest AGN at these distances .
More photons were acquired in a few minutes with Swift -XRT than XMM- Newton and Chandra obtained in \sim 300 ks of pointed observations of z > 5 AGN .
This observation is a clear demonstration of concept for efficient X-ray studies of the high- z IGM with large area , high-resolution X-ray detectors , and shows that early-phase GRBs are the only backlighting bright enough for X-ray absorption studies of the IGM at high redshift .