The startling discovery of ( ) 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 anti-bias 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 } \sim 0.3 c .
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 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 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 .