Testing the Possible Intrinsic Origin of the Excess of Very Strong MgII Absorbers Along GRB Lines-of-Sight

TL;DR

This study investigates whether the excess of very strong MgII absorbers along GRB lines of sight is intrinsic to the GRBs or their environment, by comparing their properties with those of quasar absorbers at high spectral resolution.

Contribution

The paper provides a detailed comparison of MgII absorber properties between GRB and quasar sightlines, testing the intrinsic origin hypothesis with high-resolution data.

Findings

No significant differences in metallicity, ionization, or dust between the two populations.

Intrinsic models require high outflow velocities, which are unlikely.

Results suggest similar processes produce MgII absorbers in both environments.

Abstract

The startling discovery of Prochter et al. (2006) that the frequency of very strong (W_r(2796)>1 A) MgII absorbers along gamma-ray burst (GRB) lines of sight ([dN/dz]_{GRB} = 0.90) is more than three times the frequency along quasar lines of sight ([dN/dz]_{QSO} = 0.24), over similar redshift ranges, has yet to be understood. We reconsider the possibility that the excess of very strong MgII absorbers toward GRBs is intrinsic either to the GRBs themselves or to their immediate environment, and associated with bulk outflows with velocities as large as v_{max} ~ 0.3c. In order to examine this hypothesis, we accumulate a sample of 27 W_r(2796) > 1 A absorption systems found toward 81 quasars, and compare their properties to those of 9 W_r(2796)>1 A absorption systems found toward 6 GRBs; all systems have been observed at high spectral resolution (R = 45,000) using the Ultraviolet and Visual Echelle Spectrograph on the Very Large Telescope. We make multiple comparisons of the absorber properties across the two populations, testing for differences in metallicity, ionization state, abundance patterns, dust abundance, kinematics, and phase structure. We find no significant differences between the two absorber populations using any of these metrics, implying that, if the excess absorbers toward GRB lines of sight are indeed intrinsic, they must be produced in a process which has strong similarities to the processes yielding strong MgII systems in association with intervening galaxies. Although this may seem a priori unlikely, given the high outflow velocities required for any intrinsic model, we note that the same conclusion was reached, recently, with respect to the narrow absorption line systems seen in some quasars.