Direct Observational Test Rules Out Small MgII Absorbers

TL;DR

This study tests whether small MgII absorbers can explain the higher incidence of MgII absorption in GRB sightlines and finds observational evidence against the small cloudlet hypothesis, supporting larger absorber sizes.

Contribution

It provides observational constraints that rule out the small MgII cloudlet model as an explanation for the excess of absorbers in GRB sightlines.

Findings

No anomalous absorption profiles observed, constraining small cloud models.

Distribution of MgII equivalent widths matches larger absorber scenarios.

Small MgII cloudlet hypothesis is ruled out by the data.

Abstract

Recent observations suggest the incidence of strong intervening MgII absorption systems along the line-of-sight to gamma ray burst (GRB) afterglows is significantly higher than expected from analogous quasar sightlines. One possible explanation is a geometric effect, arising because MgII absorbers only partially cover the quasar continuum regions, in which case MgII absorbers must be considerably smaller than previous estimates. We investigate the production of abnormal absorption profiles by partial coverage and conclude that the lack of any known anomalous profiles in observed systems, whilst constraining, cannot on its own rule out patchy MgII absorbers. In a separate test, we look for differences in the distribution function of MgII equivalent widths over quasar continuum regions and CIII] emission lines. We show that these anomalies should be observable in any scenario where MgII absorbers are very small, but they are not present in the data. We conclude that models invoking small MgII cloudlets to explain the excess of absorbers seen towards GRBs are ruled out.