Outshining the Quasars at Reionization: The X-Ray Spectrum and Light Curveof the Redshift 6.29 Gamma-Ray Burst GRB 050904

Gamma-ray burst (GRB) 050904 is the most distant X-ray source known, atz=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 minutesto at least half a day. The spectra soften from a power-law with photon indexGamma=1.2 to 1.9, and are well-fit by an absorbed power-law with possibleevidence of large intrinsic absorption. There is no evidence for discretefeatures, in spite of the high signal-to-noise ratio. In the days after theburst, GRB 050904 was by far the brightest known X-ray source at z>4. In thefirst minutes after the burst, the flux was >10^{-9} erg cm^-2 s^-1 in the0.2-10keV band, corresponding to an apparent luminosity >10^5 times larger thanthe brightest AGN at these distances. More photons were acquired in a fewminutes with Swift-XRT than XMM-Newton and Chandra obtained in ~300 ks ofpointed observations of z>5 AGN. This observation is a clear demonstration ofconcept 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 onlybacklighting bright enough for X-ray absorption studies of the IGM at highredshift.Comment: Accepted for publication in ApJL. 5 pages with emulateapj, 3 figure