Mg II absorbers along GRB lines of sight

Abstract

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 A). 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.