VLT/X-shooter spectroscopy of the GRB 090926A afterglow^⋆

¹ INAF – Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monteporzio Catone, Italy
e-mail: delia@mporzio.astro.it
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

Received: 15 June 2010
Accepted: 29 July 2010

Abstract

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 N_H/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 E_B−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.