Testing the gravitational lensing explanation for the MgII problem in GRBs

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.

v2 includes major updates in response to referee's comments. 15 pages, 10 figures, accepted for publication in ApJ

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• The SLUGGS Survey: Breaking degeneracies between dark matter, anisotropy and the IMF using globular cluster subpopulations in the giant elliptical NGC 5846
• An Independent Measurement of the Incidence of MgII Absorbers along Gamma-Ray Burst Sightlines: the End of the Mystery?
• Multi-messenger Gravitational Lensing
• Constraints on the relative sizes of intervening Mg II-absorbing clouds and quasar emitting regions
• On the significance of the excess number of strong MgII absorbers observed towards Gamma-ray bursts
• Clustering of galaxies around GRB sight-lines