Volume 503, Number 3, September I 2009
|Page(s)||771 - 781|
|Published online||02 July 2009|
Statistics and characteristics of MgII absorbers along GRB lines of sight observed with VLT-UVES
University Paris 7, APC, UMR7164 CNRS, 10 rue Alice Domon et Léonie Duquet, 75205 Paris Cedex 13, France e-mail: firstname.lastname@example.org
2 University Paris 6, Institut d'Astrophysique de Paris, UMR7095 CNRS, 98bis Boulevard Arago, 75014 Paris, France
3 School of Physical Sciences and NCPST, Dublin City University, Dublin 9, Ireland
4 European Southern Observatory, Alonso de Córdova 3107, Casilla 19001, Vitacura, Santiago 19, Chile
5 Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen, Juliane Marie Vej 30, 2100 Copenhagen, Denmark
6 School of Cosmic Physiscs, DIAS, 31 Fitzwilliam Street, Dublin 2, Ireland
Accepted: 12 June 2009
We analyze the properties of Mg ii absorption systems detected along the sightlines toward GRBs using a sample of 10 GRB afterglow spectra obtained with VLT-UVES over the past six years. The signal-to-noise ratio is high enough that we can extend previous studies to smaller equivalent widths (typically Wr > 0.3 Å). Over a pathlength of , we detect 9 intervening Mg ii systems with Wr > 1 Å and 9 weaker MgII systems (0.3 < Wr < 1.0 Å) when about 4 and 7, respectively, are expected from observations of QSO sightlines. The number of weak absorbers is similar along GRB and QSO lines of sight, while the number of strong systems is higher along GRB lines of sight with a 2σ significance. Using intermediate and low-resolution observations reported in the literature, we increase the absorption length for strong systems to = 31.5 (about twice the path length of previous studies) and find that the number density of strong Mg ii systems is a factor of 2.1 ± 0.6 higher (about 3σ significance) toward GRBs than towards QSOs, about twice less, however, than previously reported. We divide the sample into three redshift bins and find that the number density of strong Mg ii is greater in the low-redshift bins. We investigate in detail the properties of strong Mg ii systems observed with UVES, deriving an estimate of both the H i column density and the associated extinction. Both the estimated dust extinction in strong GRB Mg ii systems and the equivalent width distribution are consistent with what is observed for standard QSO systems. We also find that the number density of (sub)-DLAs per unit redshift in the UVES sample is probably twice more than what is expected from QSO sightlines, which confirms the peculiarity of GRB lines of sight. These results indicate that neither a dust extinction bias nor different beam sizes of the sources are viable explanations for the excess. It is still possible that the current sample of GRB lines of sight is biased by a subtle gravitational lensing effect. More data and larger samples are needed to test this hypothesis.
Key words: quasars: absorption lines / gamma rays: bursts
© ESO, 2009
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