Gamma-ray bursts as X-ray depth-gauges of the Universe

Abstract

We discuss the X-ray flux of gamma-ray burst afterglows at redshifts in the range 3-30, including the effects of the intergalactic He II absorption. We point out that strong X-ray lines may form locally in burst afterglows starting minutes after the trigger. This can provide distinctive X-ray distance indicators out to the redshifts where the first generation of massive stars form.Comment: ApJ(Lett) in press 5/31/03; subm. 5/7/0