2 00 4 Probing the environment in Gamma - ray bursts : the case of an X - ray precursor , afterglow late onset and wind vs constant density profile in GRB 011121 and GRB 011211

In this paper we present BeppoSAX and XMM-Newton 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. In the November burst the spectrum of the X-ray burst is much softer than that of the preceding prompt phase, and consistent with the spectrum of the afterglow at 1 day. In addition, the tail of the X-ray burst and the light curve of the afterglow at 1 day are connected by a single power law ≈ (t − t 0) −δ X , when t 0 corresponds with the onset of the X-ray burst. These evidences suggest that the late X-ray burst represents the onset of the afterglow. A similar conclusion is drawn for the December burst. The temporal and spectral behaviour of the X-ray and optical afterglows 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 (δ X = 1.29 ± 0.04 vs δ O = 1.66 ± 0.06). 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 – 2 – the prediction of simple models of precursor activity for collapsars, its spectrum is not consistent with a black body. Finally, a substantial absorption column (N H = (7 ± 2) × 10 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.