GRB080319B: A high resolution spectroscopic view

GRB080319B reached 5th optical mag during the burst. Thanks to the VLT/UVES rapid response mode, we observed its afterglow just 8m:30s after the GRB onset when R was ∼12. This allowed us to obtain the best signal-to-noise, high resolution spectrum of a GRB afterglow ever (S/N ∼50). The spectrum is rich of absorption features belonging to the main system at z=0.937, divided in at least six components spanning a velocity range of 120 km/s. The observations caught the absorbing gas in a highly excited state, producing the strongest FeII fine structure lines ever observed in a GRB. A few hours later the optical depth of these lines was reduced by a factor of 4-20, and the optical/UV flux by a factor of ∼60. This proves that the excitation of the observed fine structure lines is due to "pumping" by the GRB UV photons. A comparison of the observed ratio between the number of photons absorbed by the excited and ground state of the FeII suggests that the six absorbers are ∼18-34kpc from the GRB site, with component I ∼2 times closer to the GRB site than components III to VI. This suggests a structured ISM in this galaxy complex.