2009A&A...503..771V

We analyze the properties of MgII 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 W_r>0.3Å). Over a pathlength of Δz∼14, we detect 9 intervening MgII systems with W_r>1Å and 9 weaker MgII systems (0.3< W_r<1.0Å) 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 Δz=31.5 (about twice the path length of previous studies) and find that the number density of strong MgII 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 MgII is greater in the low-redshift bins. We investigate in detail the properties of strong MgII systems observed with UVES, deriving an estimate of both the HI column density and the associated extinction. Both the estimated dust extinction in strong GRB MgII 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.