2014A&A...564A..38D

We present a study of the environment of the Swift long gamma-ray burst GRB120327A at z≃2.8 through optical spectroscopy of its afterglow. We analyzed medium-resolution, multi-epoch spectroscopic observations (R∼7000-12000, corresponding to ∼15-23km/s, S/N=15-30 and wavelength range 3000-25000Å) of the optical afterglow of GRB120327A, taken with X-shooter at the VLT 2.13 and 27.65 hr after the GRB trigger. The first epoch spectrum shows that the ISM in the GRB host galaxy at z=2.8145 is extremely rich in absorption features, with three components contributing to the line profiles. The hydrogen column density associated with GRB120327A has logN_H/cm2=22.01±0.09, and the metallicity of the host galaxy is in the range [X/H]=-1.3 to -1.1. In addition to the ground state lines, we detect absorption features associated with excited states of CII, OI, SiII, FeII, and NiII, which we used to derive information on the distance between the host absorbing gas and the site of the GRB explosion. The variability of the FeII λ2396 excited line between the two epochs proves that these features are excited by the GRB UV flux. Moreover, the distance of component I is found to be d_I=200⁺¹⁰⁰_–60pc, while component II is located closer to the GRB, at d_II=100⁺⁴⁰_–30pc. These values are among the lowest found in GRBs. Component III does not show excited transitions, so it should be located farther away from the GRB. The presence of H₂ molecules is firmly established, with a molecular fraction f in the range f=4x10^–7-10^–4. This particularly low value can be attributed to the small dust content. This represents the third positive detection of molecules in a GRB environment.