An Independent Measurement of the Incidence of MgII Absorbers along
  Gamma-Ray Burst Sightlines: the End of the Mystery?
  In 2006, Prochter et al. reported a statistically significant enhancement of
very strong Mg II absorption systems intervening the sightlines to gamma-ray
bursts (GRBs) relative to the in- cidence of such absorption along quasar
sightlines. This counterintuitive result, has inspired a diverse set of
astrophysical explanations (e.g. dust, gravitational lensing) but none of these
has obviously resolved the puzzle. Using the largest set of GRB afterglow
spectra available, we reexamine the purported enhancement. In an independent
sample of GRB spectra with a survey path 3 times larger than Prochter et al.,
we measure the incidence per unit redshift of $\geq 1$\AA rest-frame equivalent
width Mg II absorbers at $z \approx 1$ to be l(z)= 0.18 $\pm$ 0.06. This is
fully consistent with current estimates for the incidence of such absorbers
along quasar sightlines. Therefore, we do not confirm the original enhancement
and suggest those results suffered from a statistical fluke. Signatures of the
original result do remain in our full sample (l(z) shows an $\approx 1.5$
enhancement over l(z)QSO), but the statistical significance now lies at
$\approx 90%$ c.l. Restricting our analysis to the subset of high-resolution
spectra of GRB afterglows (which overlaps substantially with Prochter et al.),
we still reproduce a statistically significant enhancement of Mg II absorption.
The reason for this excess, if real, is still unclear since there is no
connection between the rapid afterglow follow-up process with echelle (or
echellette) spectrographs and the detectability of strong Mg II doublets. Only
a larger sample of such high-resolution data will shed some light on this
matter.
