A possible gravitational lensing explanation for the excess of strong Mgii absorbers in gamma-ray burst afterglow spectra

Gamma-ray burst (GRB) afterglows offer a probe of the intergalactic medium (IGM) out to high redshift which complements observations along more abundant quasar lines of sight. Although both quasars and GRB afterglows should provide a priori random sightlines through the intervening IGM, it has been observed that strong Mgii absorbers are twice as likely to be found along sightlines towards GRBs. In this paper we study the effect of gravitational lensing by galaxies and their surrounding mass distributions on the statistics of Mgii absorption. We find that the multiband magnification bias could be very strong in the spectroscopic GRB afterglow population and that gravitational lensing is able to explain the discrepancy in density of absorbers, for plausibly steep luminosity functions. The model makes the prediction that to explain the Mgii absorber excess, approximately 20-60 per cent of the spectroscopic afterglow sample (i.e. ~5-15 of 26 sources) would need to have been multiply imaged, implying that many of these GRBs were repeating bursts. We show that despite this large lensing fraction it is likely that no repeating bursts would yet have been identified by chance owing to the finite sky coverage of GRB searches, while the required multiple image fraction is currently not ruled out by direct observations of the host galaxies. A confirmation of the lensing hypothesis would allow the GRB luminosity function to be constrained down to otherwise inaccessibly faint levels, with potential consequences for GRB models. On the other hand, should multiple images be shown to be absent among the spectroscopic afterglow sample, then gravitational lensing by galaxies will be ruled out as an explanation for the observed Mgii absorber excess.