Testing the gravitational lensing explanation for the MgII problem in
  GRBs
  Sixty percent of gamma-ray bursts (GRBs) reveal strong MgII absorbing
systems, which is a factor of ~2 times the rate seen along lines-of-sight to
quasars. The discrepancy in the covering factor is most likely to be the result
of either quasars being obscured due to dust, or the consequence of many GRBs
being strongly gravitationally lensed. We analyze observations of GRBs that
show strong foreground MgII absorption. We Monte-Carlo the distances between
foreground galaxies in the HUDF and lines of sight distributed randomly within
a radius derived from the covering factor, and find that galaxies are located
systematically closer to the position of the observed GRBs than expected for
random lines of sight. This over-density at small impact parameters is
statistically more robust than the well known excess of MgII absorbers among
GRB afterglow spectra, and presents a new puzzle for MgII absorption studies.
The over-density cannot be explained by obscuration in the GRB sample, but is a
natural consequence of gravitational lensing. Upon examining the particular
configurations of galaxies near a sample of GRBs with strong MgII absorption,
we find several intriguing lensing candidates. Our results suggest that lensing
provides a viable contribution to the observed enhancement of strong MgII
absorption along lines of sight to GRBs, and we outline the future observations
required to test this hypothesis conclusively.
