2009ApJ...697..345C

The startling discovery by Prochter et al. that the frequency of very strong (W_r(2796)>1 Å) Mg II 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. In particular, explanations appealing to dust antibias in quasar samples, partial covering of the quasar sources, and gravitational-lensing amplification of the GRBs have all been carefully examined and found wanting. We therefore reconsider the possibility that the excess of very strong Mg II 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 Å absorption systems found toward 81 quasars, and compare their properties to those of 8 W_r(2796) > 1 Å absorption systems found toward six 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 of absorbers along GRB lines of sight are indeed intrinsic, they must be produced by a process which has strong similarities to the processes yielding strong Mg II systems associated 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.