The X-ray spectrum and lightcurve of the redshift 6.29 γ-Ray Burst GRB 050904.

A γ-ray burst (GRB) has been found with a redshift comparable to the most distant quasars and galaxies: GRB 050904 at z = 6.29 ± 0.01, making it the most distant X-ray source known. The X-ray lightcurve is not a power-law like many afterglows, but is dominated by large amplitude variability from a few minutes to at least half a day. The spectra soften during this time from a power-law with photon index Γ = 1.2 to 1.9. The spectra are well-described by an absorbed power-law with possible evidence of very large intrinsic absorption. There is no evidence for discrete features. This is in spite of the spectrum’s very high signal-to-noise ratio, since GRB 050904 was extraordinarily bright in X-rays. In the first days after the burst, it was by far the brightest known X-ray source at z > 4. In the first minutes after the burst, the X-ray flux was > 10-9 erg cm-2 s-1 in the 0.2–10 keV band, corresponding to an apparent luminosity between 10 5 and 106 times greater than the brightest X-ray quasars at similar distances. More photons were acquired in the first minutes with Swift-XRT than XMM-Newton and Chandra have obtained in ∼ 300 ks of pointed observations of z > 5 AGN. The huge X-ray fluence detected from GRB 050904 is a clear demonstration of concept for efficient X-ray studies of the high-z IGM with new large area, high-resolution X-ray detectors, and shows that GRBs in their early phases are the only backlighting bright enough for X-ray absorption studies of the intervening matter at high redshift. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]