THE PROMPT, HIGH-RESOLUTION SPECTROSCOPIC VIEW OF THE “NAKED-EYE” GRB080319B

Abstract

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.