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

Abstract

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..