Solving the conundrum of intervening strong Mg II absorbers towards gamma-ray bursts and quasars

Previous studies have shown that the incidence rate of intervening strong Mg II absorbers towards gamma-ray bursts (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 Mg II absorbers with rest-frame equivalent widths W_r^(λl2796>1) Å towards GRBs and quasars in the redshift range 0.1 ≲ z ≲ 5. With large and similar sample sizes, and path length coverages of Δz = 57.8 and 254.4 for GRBs and quasars, respectively, the incidences of intervening absorbers are consistent within 1σ uncertainty levels at all redshifts. For absorbers at z < 2.3, the incidence towards GRBs is a factor of 1.5 ± 0.4 higher than the expected number of strong Mg II absorbers in Sloan Digital Sky Survey (SDSS) quasar spectra, while for quasar absorbers observed with X-shooter we find an excess factor of 1.4 ± 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 Mg II catalogues and high-spectral-resolution studies. The rest-frame equivalent-width distribution also agrees to within 1σ uncertainty levels between the GRB and quasar samples. Intervening strong Mg II absorbers towards GRBs are therefore neither unusually frequent, nor unusually strong.