The Prompt, High-Resolution Spectroscopic View of the “Naked-Eye” Grb080319b

GRB080319B reached fifth optical magnitude during the burst prompt emission. Thanks to the Very Large Telescope (VLT)/Ultraviolet and Visual Echelle Spectrograph (UVES) rapid response mode, we observed its afterglow just 8m:30s after the gamma-ray burst (GRB) onset when the magnitude was R ∼ 12. This allowed us to obtain the best signal-to-noise (S/N), high-resolution spectrum of a GRB afterglow ever (S/N per resolution element ∼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 total velocity range of 100 km s −1 . The VLT/UVES observations caught the absorbing gas in a highly excited state, producing the strongest Fe ii 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 state and those in the Fe ii ground state suggests that the six absorbers are ∼2–6 kpc from the GRB site, with component I ∼ 3 times closer to the GRB site than components III–VI. Component I is characterized also by the lack of Mg i absorption, unlike all other components. This may be both due to a closer distance and a lower density, suggesting a structured interstellar matter in this galaxy complex.