Strong Mg II Systems in Quasar and Gamma-Ray Burst Spectra

The incidence of strong Mg II systems in gamma-ray burst (GRB) spectra is a few times higher than in quasar (QSO) spectra. We investigate several possible explanations for this effect, including dust obscuration bias, clustering of the absorbers, different beam sizes of the sources, multiband magnification bias of GRBs, and association of the absorbers with the GRB event or the circumburst environment. We find that (1) the incidence rate of Mg II systems in QSO spectra could be underestimated by a factor of 1.3-2 due to dust obscuration; (2) the equivalent width distribution of the Mg II absorbers along GRBs is consistent with that observed along QSOs, thus suggesting that the absorbers are more extended than the beam sizes of the sources; (3) on average, GRB afterglows showing more than one Mg II system are a factor of 1.7 brighter than the others, suggesting a lensing origin of the observed discrepancy; (4) gravitational lensing (in different forms, from galaxy lensing to microlensing) can bias high the counts of Mg II systems along GRBs if the luminosity functions of the prompt gamma-ray emission and of the optical afterglows have a mean faint-end slope approaching -5/3 to -2; and (5) some of the absorbers can be associated with the circumburst environment or produced by supernova remnants unrelated to the GRB event itself but lying in the same star-forming region. With the possible exception of magnification bias, it is unlikely that one of these effects on its own can fully account for the observed counts. However, the combined action of some of them can substantially reduce the statistical significance of the discrepancy.