4 Conclusions

The reverse shock propagating into the wind following its interaction with the ISM has been shown to produce, for a sufficiently dense medium, a late X-ray emission. In the case of GRB 960720, whose gamma-ray properties are well explained by internal shocks, taking into account this additional contribution greatly improves the X-ray profiles (the power-law between the profile width and the energy is reproduced) and the spectrum.

It is now important to study the case of more complex environments. In hypernova models, the progenitor is a massive Wolf-Rayet star with a dense wind ($\dot{M} \sim 3\ 10^{-5}\ M_{\odot}/{\rm yr}$ and $v_{\infty} \sim 2000\ {\rm km~s}^{-1}$ typically) leading to high densities ($n \sim \frac{\dot{M}}{4 \pi r^{2} m_{\rm p} v_{\infty}}$)near the source. Our first calculations show that in this case even the gamma-ray profile could be strongly affected by the reverse shock, which may represent a potential problem.