Testing density profile in Gamma-Ray Burst

The GRB afterglow evolution strictly depends on the properties of the external medium in which the fireball expands. Studying afterglow emission, we tried to constrain the nature of the medium. We studied afterglow evolution either in a homogeneous external medium and in a wind ejected by the GRB progenitor (probably a Wolf-Rayet Star) during the final phases of its life. We also extended this analysis to a medium characterized by a discontinuous density profile. Such discontinuities can be due to a variable activity of wind emission by massive star progenitors or to the interaction of the wind with the external medium. We numerically integrated the equations of the Standard Fireball Model, and applied it to GRB011121 and XRF011030, two events detected by the BeppoSAX satellite. These two objects show a late X-ray burst (reburst) at about few hundred seconds after the main pulse. We tried to explain the reburst as the onset of the external shock and we found that it can be successfully explained under this assumption if the fireball is expanding in a thick shell.