The Prompt X-Ray Emission of GRB 011211: Possible Evidence of a Transient Absorption Feature

We report observation results of the prompt X- and gamma-ray emission from GRB 011211. This event was detected with the Gamma-Ray Burst Monitor and one of the wide-field cameras aboard the BeppoSAX satellite. The optical counterpart of the gamma-ray burst was soon identified and its redshift determined (z = 2.140), while the X-ray afterglow emission was detected with the XMM-Newton satellite. Evidence of soft X-ray emission lines was reported by Reeves and colleagues but not confirmed by other authors. In investigating the spectral evolution of the prompt emission, we find the possible evidence of a transient absorption feature at 6.9 keV during the rise of the primary event. The significance of the feature is derived with nonparametric tests and numerical simulations, finding a chance probability that ranges from 3 × 10^-3 down to 4 × 10^-4. The feature shows a Gaussian profile and an equivalent width of 1.2 keV. We discuss our results and their possible interpretation.