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We analyse the properties of Mg II absorption systems detected along the sightlines toward GRBs using VLT-UVES GRB afterglow spectra. We extend previous studies to smaller equivalent widths (typically Wr>0.3 Å). We increase the absorption length for strong systems to Δz = 31.5 using intermediate resolution observations reported in the literature. We find that the number of weak systems is similar in GRBs and QSOs while the number of strong systems is larger of a factor ∼2 along GRB lines of sight, about twice smaller than previously reported. We find that also the number density of (sub)-DLAs per unit redshift in the UVES sample is probably twice larger that what is expected from QSO sightlines. Both the estimated dust extinction in strong GRB Mg II systems and the equivalent width distributions are consistent with what is observed in QSO-DLAs. All this further supports the idea that the current sample of GRB lines of sight could be biased by a gravitational lensing effect. [ABSTRACT FROM AUTHOR]

We present high-resolution spectroscopic observations of GRB 060418, obtained with VLT/UVES. A sequence of spectra covering 330–670 nm were acquired at 11, 16, 25, 41 and 71 minutes (mid-exposure) after the Swift trigger, with a resolving power of 7 km s-1, and a signal-to-noise ratio of 10–15. This time-series clearly shows evidence for time variability of permitted transitions involving FeII fine-structure levels and metastable levels of both FeII and NiII, at the host-galaxy redshift z = 1.490. We model the observed evolution of the level populations with three different excitation mechanisms: collisions, excitation by infra-red photons, and fluorescence following excitation by ultraviolet photons. Our data allow us to reject the collisional and IR excitation scenarios. The UV pumping model, in which the GRB afterglow UV photons excite a cloud of atoms with column densities log N(FeII) = 14.75±0.06, log N(NiII) = 13.84±0.02, at a distance of 1.7±0.2 kpc provides an excellent fit. The success of our UV pumping modeling implies that no significant amount of neutral material (FeII, NiII or HI) is present at distances smaller than roughly 1.7 kpc. Therefore, the neutral gas responsible for the large HI column densities typically observed along GRB sightlines may not be located in the immediate environment of the GRB. [ABSTRACT FROM AUTHOR]