Outshining the quasars at reionization: The X-ray spectrum and light curve of the redshift 6.29 gamma-ray burst GRB 050904

Gamma-ray burst (GRB) 050904 is the most distant X-ray source known, at z = 6.295, comparable to the farthest AGNs 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 F = 1.2-1.9 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 ergs cm^-2 s^-1 in the 0.2-10 keV band, corresponding to an apparent luminosity >10 ⁵ times larger than the brightest AGNs 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 AGNs. 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.