UVES/VLT high resolution spectroscopy of GRB 050730 afterglow: probing the features of the GRB environment
D'Elia, V. and Fiore, F. and Meurs, E.J.A. and Chincarini, G. and Melandri, A. and Norci, Laura and Pellizza, L. and Perna, R. and Piranomonte, S. and Sbordone, L. and Stella, L. and Tagliaferri, G. and Vergani, Susanna D. and Ward, P.A. and Angelini, L. and Antonelli, L.A. and Burrows, D.N. and Campana, M. and Capalbi, M. and Cimatti, A. and Costa, E. and Cusumano, G. and Della Valle, M. and Filliatre, P. and Fontana, A. and Frontera, F. and Fugazza, D. and Gehrels, N. and Giannini, T. and Giommi, P. and Goldoni, P. and Guetta, D. and Israel, G. and Lazzati, D. and Malesani, D. and Marconi, G. and Mason, K. and Mereghetti, S. and Mirabel, F. and Molinari, E. and Moretti, A. and Nousek, J. and Perri, M. and Piro, L. and Stratta, G. and Testa, V. and Vietri, M. (2007) UVES/VLT high resolution spectroscopy of GRB 050730 afterglow: probing the features of the GRB environment. Astronomy & Astrophysics, 467 (2). pp. 629-639. ISSN 0004-6361
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Aims. The aim of this paper is to study the Gamma Ray Burst (GRB) environment through the analysis of the optical absorption features due to the gas surrounding the GRB.
Methods. To this purpose we analyze high resolution spectroscopic observations (R=20000-45000, corresponding to 14 km/s at 4200Å and 6.6 km/s at 9000Å) of the optical afterglow of GRB050730, obtained with UVES@VLT ∼ 4 hours after the GRB trigger.
Results. The spectrum shows that the ISMof the GRB host galaxy at z = 3.967 is complex, with at least five components contributing to the main absorption system. We detect strong CII*, Si II*, OI* and Fe II* fine structure absorption lines associated to the second and third component.
Conclusions. For the first three components we derive information on the relative distance from the site of the GRB explosion. Component 1, which has the longest wavelength, highest positive velocity shift, does not present any fine structure nor low ionization lines; it only shows very high ionization features, such as CIV and OVI, suggesting that this component is very close to the GRB site. From the analysis of low and high ionization lines and fine structure lines, we find evidences that the distance of component 2 from the site of the GRB explosion is 10-100 times smaller than that of component 3. We evaluated the mean metallicity of the z=3.967 system obtaining values ≈ 10−2 of the solar metallicity or less. However, this should not be taken as representative of the circumburst medium, since the main contribution to the hydrogen column density comes from the outer regions of the galaxy while that of the other elements presumably comes from the ISMcloser to the GRB site. Furthermore, difficulties in evaluating dust depletion correction can modify significantly these values.
The mean [C/Fe] ratio agrees well with that expected by single star-formation event models. Interestingly the [C/Fe] of component 2 is smaller than that of component 3, in agreement with GRB dust destruction scenarios, if component 2 is closer than component 3 to the GRB site.
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