Volume 523, November-December 2010
|Number of page(s)||11|
|Section||Interstellar and circumstellar matter|
|Published online||15 November 2010|
VLT/X-shooter spectroscopy of the GRB 090926A afterglow⋆
INAF – Osservatorio Astronomico di Roma, via Frascati 33, 00040
2 ASI-Science Data Center, via Galileo Galilei, 00044 Frascati, Italy
3 Dark Cosmology Center, Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, 2100 Copenhagen, Denmark
4 INAF – Osservatorio Astronomico di Brera, via E. Bianchi 46, 23807 Merate ( LC), Italy
5 Laboratoire Astroparticule et Cosmologie, 10 rue A. Domon et L. Duquet, 75205 Paris Cedex 13, France
6 DSM/IRFU/Service D’Astrophysique, CEA-Saclay, 91191 Gif-sur-Yvette, France
7 Center for Astrophysics and Cosmology, Science Institute, University of Iceland, Dunhagi 5, 107 Reykjavik, Iceland
8 INAF, Osservatorio Astronomico di Trieste, via Tiepolo 11, 34143 Trieste, Italy
9 The Oskar Klein Center, Department of Astronomy, AlbaNova, Stockholm University, 106 91 Stockholm, Sweden
10 University Paris 7, APC, Lab. Astroparticule et Cosmologie, UMR7164 CNRS, 10 rue Alice Domon et Lonie Duquet, 75205 Paris Cedex 13, France
11 Laboratoire Galaxies Etoiles Physique et Instrumentation, Observatoire de Paris, 5 place Jules Janssen, 92195 Meudon, France
12 Department of Physics and Astronomy, University of Leicester, Leicester LE1 7RH, UK
13 Astronomical Institute “Anton Pannekoek”, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands
14 Department of Physics, University of Warwick, Coventry, CV4 7AL, UK
15 Scuola Normale Superiore, Piazza dei Cavalieri 7, 56126 Pisa, Italy
Accepted: 29 July 2010
Aims. The aim of this paper is to study the environment and intervening absorbers of the gamma-ray burst GRB 090926A through analyzing optical spectra of its afterglow.
Methods. We analyzed medium-resolution spectroscopic observations (R = 10000, corresponding to 30 km s-1, S/N = 15−30 and wavelength range 3000−25000) of the optical afterglow of GRB 090926A, taken with X-shooter at the VLT ~ 22 h after the GRB trigger.
Results. The spectrum shows that the ISM in the GRB host galaxy at z = 2.1071 is rich in absorption features, with two components contributing to the line profiles. In addition to the ground state lines, we detect C ii, O i, Si ii, Fe ii, and Ni ii-excited absorption features, which we used to derive information on the distance between the host absorbing gas and the site of the GRB explosion. The distance of component I is found to be 2.40 ± 0.15 kpc, while component II is located far away from the GRB, possibly at ~5 kpc. These values are compatible with those found for other GRBs. The hydrogen column density associated to GRB 090926A is log NH/cm-2 = 21.60 ± 0.07, and the metallicity of the host galaxy is in the range [X/H] = −2.5 to −1.9 with respect to the solar values, i.e., among the lowest values ever observed for a GRB host galaxy. A comparison with galactic chemical evolution models has suggested that the host of GRB090926A is likely to be a dwarf-irregular galaxy. No emission lines were detected, but a Hα flux in emission of 9 × 10-18 erg s-1 cm-2 (i.e., a star-formation rate of 2 M⊙ yr-1), which is typical of many GRB hosts, would have been detected in our spectra, and thus emission lines are well within the reach of X-shooter. We put an upper limit to the H molecular fraction of the host galaxy ISM, which is f < 7 × 10-7. The continuum has been fitted assuming a power-law spectrum, with a spectral index of . The best fit does essentially not require any intrinsic extinction because EB−V < 0.01 mag adopting a SMC extinction curve. Finally, the line of sight of GRB 090926A presents four weak intervening absorption systems in the redshift range 1.24 < z < 1.95.
Key words: ISM: abundances / ISM: atoms / galaxies: star formation / line: profiles / techniques: spectroscopic / atomic processes
© ESO, 2010
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