Solving the conundrum of intervening strong MgII absorbers towards GRBs and quasars

TL;DR

This study finds that the incidence of strong MgII absorbers towards GRBs and quasars is consistent when using uniform, large samples, resolving previous discrepancies and showing no unusual frequency or strength of these absorbers.

Contribution

The paper demonstrates that previous apparent excesses of MgII absorbers towards GRBs are due to sample biases, providing a unified analysis with large, uniform datasets.

Findings

Incidence rates of MgII absorbers are consistent between GRBs and quasars within uncertainties.

No significant difference in the strength distribution of MgII absorbers between GRBs and quasars.

Discrepancies are due to differences in data quality and cataloging, not intrinsic differences.

Abstract

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.