Preprint typeset using LATEX style emulateapj v. 5/2/11 ON THE SIGNIFICANCE OF THE EXCESS NUMBER OF STRONG MGII ABSORBERS OBSERVED TOWARDS GAMMA-RAY BURSTS

The number of strong (equivalent width> 1Å) Mgii absorbers observed towards Gamma-ray bursts (GRBs) has been found to be statistically larger than the number of strong absorbers towards quasi-stellar objects (QSOs). We formalise this “Mgii problem ” and present a detailed explanation of the statistical tools required to assess the significance of the discrepancy. We find the problem exists at the 4σ level for GRBs with high-resolution spectra. It has been suggested that the discrepancy can be resolved by the combination of a dust obscuration bias towards QSOs, and a strong gravitational lens-ing bias towards GRBs. We investigate one of the two most probable lensed GRBs that we presented in our previous work (GRB020405; Rapoport et al.) and find it not to be strongly gravitationally lensed, constraining the percentage of lensed GRBs to be < 35 % (2σ). Dust obscuration of QSOs has been estimated to be a significant effect with dusty Mgii systems removing ∼20 % of absorbed objects from flux-limited QSO samples. We find that if ∼30 % of the strong Mgii systems towards QSOs are missing from the observed samples, then GRBs and QSOs would have comparable numbers of ab-sorbers per unit redshift. Thus, gravitational lensing bias is likely to make only a modest contribution to solving the Mgii problem. However, if the dust obscuration bias has been slightly underestimated, the Mgii problem would no longer persist