2011MNRAS.410.1611C

Gamma-ray bursts (GRBs) are the most violent explosions in the Universe. Long-duration GRBs are associated with the collapse of massive stars, rivalling their host galaxies in luminosity. The discovery of the most distant spectroscopically confirmed object in the Universe, GRB090423, opened a new window on the high-redshift Universe, making it possible to study the cosmic reionization epoch and the preceding dark ages, as well as the generation of the first stars (Population III) using GRBs. Obviously this enables a wealth of new studies using the near-infrared (NIR) characteristics of GRB afterglows. Here we explore a different path, focusing on the next generation of X-ray missions with large-area-focusing telescopes and fast-repointing capabilities. We found that X-ray data can complement NIR observations and for the brightest GRBs can provide an accurate and independent redshift determination. Metallicity studies can also be carried out profitably once the redshift is known. Finally we discuss observational signatures of GRBs arising from Population III stars in the X-ray band.