Abstract

There are currently a few cases where a supernova was associated with a Gamma-Ray Burst, proving that GRBs arise from the death of massive stars. Other lines of evidence supporting this conclusion are the spatial location of bursts in the host galaxy, the detection of multiple high velocity absorption lines in GRB 021004, and of X-ray emission lines and edges for a few afterglows. Massive stars drive powerful winds, shaping the circumstellar medium up to tens of parsecs. Modeling of the broadband afterglow emission with a relativistic fireball interacting with the circumburst medium, yields estimations of its particle density. The resulting values, ranging from 0.1 cm-3 to 50 cm-3, are consistent with the density of the wind from a Wolf-Rayet star at the typical distance (0.1 ÷ 1 pc) where the afterglow is expected to occur. The r˗2 density profile expected around a massive star is consistent with the results of afterglow modeling in a majority of cases; nevertheless there are a few afterglows for which a homogeneous medium accommodates much better the sharpness of the optical light-curve break. Afterglow modeling also shows that the kinetic energy of GRB jets spans the range 1050 and 3 × 1051 ergs, i.e. slightly less than that of the supernova ejecta. The burst γ-ray energy output, corrected for collimation, has a similar range.