Casting light on the 'anomalous' statistics of Mg II absorbers toward Gamma-Ray Burst afterglows: the incidence of weak systems

TL;DR

This study analyzes the incidence of weak Mg II absorption systems in GRB afterglow spectra, finding their occurrence rate is consistent with quasars, contrasting with the higher incidence of strong systems in GRBs, and suggesting lensing as a possible explanation.

Contribution

It provides the first statistical comparison of weak Mg II absorber incidence between GRBs and QSOs, clarifying the nature of the previously observed discrepancy for strong systems.

Findings

Weak Mg II systems have similar incidence in GRBs and QSOs.

Strong Mg II systems are more common toward GRBs than QSOs, by a factor of ~4.

Lensing amplification could explain the excess of strong Mg II systems in GRBs.

Abstract

We revisit echelle spectra (spectral resolution R ~ 40000) of 8 Gamma-Ray Burst afterglows to obtain the incidence (dN/dz) of weak intervening Mg II systems at a mean redshift of <z> = 1.5. We show that dN/dz of systems having restframe equivalent widths 0.07 A < W_r(MgII) < 1 A toward GRBs is statistically consistent with the incidence toward QSOs. Our result is in contrast to the results for Mg II systems having W_r > 1 A, where dN/dz toward GRBs has been found to be larger than toward QSOs by a factor of ~ 4. We confirm the overdensity albeit at a factor of ~ 3 only. This suggests that any explanation for the GRB/QSO discrepancy, be it intrinsic to the absorbers or a selection effect, should be inherent only to the galaxies that host strong absorbers in the line-of-sight to GRBs. We argue that, of all scenarios that have been proposed, lensing amplification is the one that could explain the strong Mg II enhancement while allowing for no significant enhancement in the weak absorbers.