Solving the conundrum of intervening strong MgII absorbers towards GRBs
  and quasars
  Previous studies have shown that the incidence rate of intervening strong
MgII absorbers towards GRBs were a factor of 2 - 4 higher than towards quasars.
Exploring the similar sized and uniformly selected legacy data sets XQ-100 and
XSGRB, each consisting of 100 quasar and 81 GRB afterglow spectra obtained with
a single instrument (VLT/X-shooter), we demonstrate that there is no
disagreement in the number density of strong MgII absorbers with rest-frame
equivalent widths $W_r^{2796} >$ 1 {\AA} towards GRBs and quasars in the
redshift range 0.1 < z < 5. With large and similar sample sizes, and path
length coverages of $\Delta$z = 57.8 and 254.4 for GRBs and quasars,
respectively, the incidences of intervening absorbers are consistent within 1
sigma uncertainty levels at all redshifts. For absorbers at z < 2.3 the
incidence towards GRBs is a factor of 1.5$\pm$0.4 higher than the expected
number of strong MgII absorbers in SDSS quasar spectra, while for quasar
absorbers observed with X-shooter we find an excess factor of 1.4$\pm$0.2
relative to SDSS quasars. Conversely, the incidence rates agree at all
redshifts with reported high spectral resolution quasar data, and no excess is
found. The only remaining discrepancy in incidences is between SDSS MgII
catalogues and high spectral resolution studies. The rest-frame equivalent
width distribution also agrees to within 1 sigma uncertainty levels between the
GRB and quasar samples. Intervening strong MgII absorbers towards GRBs are
therefore neither unusually frequent, nor unusually strong.
