  In this paper we present BeppoSax and XMM observations of two long Gamma-ray
bursts, the X-ray rich event of December 11, 2001 (GRB011211) and the hard and
very bright event of November 21, 2001 (GRB011121). In both events we find
evidence of a late X-ray burst, taking place several minutes after the prompt
emission. The temporal and spectral behaviour of this phenomenon suggest that
it represents the onset of the afterglow. Broad band spectral modelling of the
afterglow indicate that the fireball evolution in the December burst takes
place in a ISM environment. On the contrary in the November burst the wind case
is revealed by an X-ray decay slower than that observed in the optical
($\delta_{\rm X}=1.29\pm0.04$ vs $\delta_{\rm O}=1.66\pm0.06$). The wind
profile should change into a constant density profile at large radii, in order
to reconcile late-time radio data with a jet. Two other results are obtained
for this burst. An X-ray burst is preceding by about 30 s the much harder GRB.
Contrary to the prediction of simple models of precursor activity for
collapsars, its spectrum is not consistent with a black body. Finally, a
substantial absorption column ($\NH=(7\pm2)\times10^{22}$ cm$^{-2}$) is
detected during the early part of the prompt emission. This is much greater
than that of the wind, and it is thus likely associated with the region
surrounding the burst.

Astrophysics