X-Ray Afterglows of Gamma-Ray Bursts
  The afterglow emission has become the main stream of Gamma-Ray burst research since its discovery three years ago. With the distance-scale enigma solved, the study of the late-time GRB emission is now the most promising approach to disclose the origin of these explosions and their relationship with the environment of the host galaxy in the early phase of the Universe. In this contribution I will review X-ray observations and their implication on our undertstanding on the GRB phenomenon. These measurements are providing a direct probe into the nature of the progenitor and a measurement of the GRB beaming properties, crucial to establish the total energy output. Some evidence of iron lines connects the GRB explosion with massive progenitors, thence with star-forming regions. Furthermore a comparison of the spectral properties with the temporal evolution indicates that the fireball expansion should not be - on average - highly collimated, with a jet angle $>10 \deg$. 
Luigi Piro (IAS/CNR, Roma)