摘要：Previous studies have shown that the incidence rate of intervening strong Mg iiabsorbers 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 iiabsorbers with rest-frame equivalent widths\hbox{$W_{\rm r}^{\lambda2796}>1$}Å towardsGRBs and quasars in the redshift range0.1 ≲z≲ 5. With large and similar sample sizes, and path length coverages ofΔz= 57.8and254.4for GRBs and quasars, respectively, the incidences of intervening absorbers are consistent within 1σuncertainty levels at all redshifts. For absorbers atz< 2.3,the incidence towards GRBs is a factor of1.5 ± 0.4higher than the expected number of strong Mg iiabsorbers in Sloan Digital Sky Survey (SDSS) quasar spectra, while for quasar absorbers observed with X-shooter we find an excess factor of1.4 ± 0.2relative 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 iicatalogues and high-spectral-resolution studies. The rest-frame equivalent-width distribution also agrees to within 1σuncertainty levels between the GRB and quasar samples. Intervening strong Mg iiabsorbers towards GRBs are therefore neither unusually frequent, nor unusually strong.
关键词：quasars: absorption lines ; gamma rays: general ; galaxies: halos
