Gamma-Ray Bursts (GRBs) are the most violent explosions in the Universe. Longduration GRBs are associated with the collapse of massive stars, rivaling theirhost galaxies in luminosity. The discovery of the most distantspectroscopically confirmed object in the Universe, GRB090423, opened a newwindow on the high redshift Universe, making it possible to study the cosmic're-ionization' epoch and the preceding dark ages, as well as the generation ofthe first stars (Population III) using GRBs. Obviously this enables a wealth ofnew studies using the near infrared (nIR) characteristics of GRB afterglows.Here we explore a different path, focusing on the next generation of X-raymissions with large area focusing telescopes and fast repointing capabilities.We found that X-ray data can complement nIR observations and for the brightestGRBs can provide an accurate and independent redshift determination.Metallicity studies can also be carried out profitably once the redshift isknown. Finally we discuss observational signatures of GRBs arising fromPopulation III stars in the X-ray band.
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