  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
0.2-10keV 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 ~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.
