Testing the gravitational lensing explanation for the MgII problem in GRBs

TL;DR

This study investigates whether gravitational lensing explains the higher incidence of MgII absorption systems in GRB lines of sight compared to quasars, suggesting lensing as a significant factor.

Contribution

It provides statistical evidence that gravitational lensing contributes to the MgII problem in GRBs, supported by galaxy over-density near GRB lines of sight.

Findings

Galaxies are closer to GRBs with MgII absorption than expected by chance.

The over-density supports gravitational lensing as a cause for MgII excess.

Lensing candidates are identified near several GRBs with strong MgII absorption.

Abstract

Sixty percent of gamma-ray bursts (GRBs) reveal strong MgII absorbing systems, which is a factor of ~2 times the rate seen along lines-of-sight to quasars. The discrepancy in the covering factor is most likely to be the result of either quasars being obscured due to dust, or the consequence of many GRBs being strongly gravitationally lensed. We analyze observations of GRBs that show strong foreground MgII absorption. We Monte-Carlo the distances between foreground galaxies in the HUDF and lines of sight distributed randomly within a radius derived from the covering factor, and find that galaxies are located systematically closer to the position of the observed GRBs than expected for random lines of sight. This over-density at small impact parameters is statistically more robust than the well known excess of MgII absorbers among GRB afterglow spectra, and presents a new puzzle for MgII absorption studies. The over-density cannot be explained by obscuration in the GRB sample, but is a natural consequence of gravitational lensing. Upon examining the particular configurations of galaxies near a sample of GRBs with strong MgII absorption, we find several intriguing lensing candidates. Our results suggest that lensing provides a viable contribution to the observed enhancement of strong MgII absorption along lines of sight to GRBs, and we outline the future observations required to test this hypothesis conclusively.