Probing the Environment in Gamma-Ray Bursts: The Case of an X-Ray Precursor, Afterglow Late Onset, and Wind Versus 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 (GRBs), the X-ray-rich event of 2001 December 11 (GRB 011211) and the hard and very bright event of 2001 November 21 (GRB 011121). 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-t₀)^-δ_X, when t₀ 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 behavior of the X-ray and optical afterglows indicate that the fireball evolution in the December burst takes place in an interstellar medium (ISM) environment. In contrast, 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 precedes the much harder GRB by about 30 s. Contrary to the prediction of simple models of precursor activity for collapsars, the precursor's spectrum is not consistent with a blackbody. Finally, a substantial absorption column [N_H=(7+/-2)×10²² 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.