A gravitational lensing explanation for the excess of strong Mg-II
  absorbers in GRB afterglow spectra
  GRB afterglows offer a probe of the intergalactic medium 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
sight-lines through the intervening IGM, it has been observed that strong Mg-II
absorbers are twice as likely to be found along sight-lines toward GRBs.
Several proposals to reconcile this discrepancy have been put forward, but none
has been found sufficient to explain the magnitude of the effect. In this paper
we estimate the effect of gravitational lensing by galaxies and their
surrounding mass distributions on the statistics of Mg-II absorption. We find
that the multi-band magnification bias could be very strong in the
spectroscopic GRB afterglow population and that gravitational lensing can
explain the discrepancy in density of absorbers, for plausibly steep luminosity
functions. The model makes the prediction that approximately 20%-60% of the
spectroscopic afterglow sample (i.e. ~ 5-15 of 26 sources) would have been
multiply imaged, and hence result in repeating bursts. We show that despite
this large lensing fraction it is likely that none would yet have been
identified by chance owing to the finite sky coverage of GRB searches. We
predict that continued optical monitoring of the bright GRB afterglow locations
in the months and years following the initial decay would lead to
identification of lensed GRB afterglows. A confirmation of the lensing
hypothesis would allow us to constrain the GRB luminosity function down to
otherwise inaccessibly faint levels, with potential consequences for GRB
models.
