X-Ray Spectroscopy of Gamma-Ray Bursts
  Observational evidence of iron absorption and emission lines in X-ray spectra of Gamma-Ray Bursts is quite compelling. I will briefly review the results, summarize different models and describe the connection with massive progenitors in star-forming regions implied by these results. This link is also supported by measurements of the X-ray absorbing gas in several GRB's, with column density consistent with that of Giant Molecular Clouds harbouring star-formation in our Galaxy, as well as by evidences gathered in other wavelengths. However, the volume density inferred by the fireball-jet model is much lower than typical of a GMC, and I will confront this with the alternative explanation of fireball expansion in a high dense medium, outlining the problems that both models have at present. Finally I will briefly summarize some results on dark GRB's, and describe the prospects of high resolution X-ray spectroscopy in getting closer to the central environment of GRB, and far in the Early Universe by using GRB as beacons to probe star and galaxy formation. 
Luigi Piro (IASF/CNR, Rome)