  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.

Astrophysics