Casting light on the 'anomalous' statistics of Mg II absorbers toward
  Gamma-Ray Burst afterglows: the incidence of weak systems
  We revisit echelle spectra (spectral resolution R ~ 40000) of 8 Gamma-Ray
Burst afterglows to obtain the incidence (dN/dz) of weak intervening Mg II
systems at a mean redshift of <z> = 1.5. We show that dN/dz of systems having
restframe equivalent widths 0.07 A < W_r(MgII) < 1 A toward GRBs is
statistically consistent with the incidence toward QSOs. Our result is in
contrast to the results for Mg II systems having W_r > 1 A, where dN/dz toward
GRBs has been found to be larger than toward QSOs by a factor of ~ 4. We
confirm the overdensity albeit at a factor of ~ 3 only. This suggests that any
explanation for the GRB/QSO discrepancy, be it intrinsic to the absorbers or a
selection effect, should be inherent only to the galaxies that host strong
absorbers in the line-of-sight to GRBs. We argue that, of all scenarios that
have been proposed, lensing amplification is the one that could explain the
strong Mg II enhancement while allowing for no significant enhancement in the
weak absorbers.
