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Astrophysical Journal 694 (2009) 332-338
The prompt, high resolution spectroscopic view of the "naked-eye" GRB080319B
V. D'Elia, F. Fiore, R. Perna, Y. Krongold, S. Covino, D. Fugazza, D. Lazzati, F. Nicastro, L. A. Antonelli, S. Campana, G. Chincarini, P. D'Avanzo, M. Della Valle, P. Goldoni1, D. Guetta, C. Guidorzi, E. J. A. Meurs, E. Molinari, L. Norci, S. Piranomonte, L. Stella, G. Stratta, G. Tagliaferri, P. Ward

GRB080319B reached 5th optical magnitude during the burst. Thanks to the VLT/UVES rapid response mode, we observed its afterglow just 8m:30s after the GRB onset when the magnitude was R ~ 12. This allowed us to obtain the best signal-to-noise, 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. 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 $\gs18-34$ kpc from the GRB site, with component I ~ 2 times closer to the GRB site than components III to VI. Component I is characterized also by the lack of Mg I absorption, unlike all other components. This may be due to a higher gas temperature, suggesting a structured ISM in this galaxy complex.
1 :  APC - UMR 7164 - AstroParticule et Cosmologie
Physique/Astrophysique/Cosmologie et astrophysique extra-galactique

Planète et Univers/Astrophysique/Cosmologie et astrophysique extra-galactique
gamma rays: bursts
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