Are Quasars Bigger than Gamma-ray Bursts? Explaining Disparate Absorption Statistics with Patchy Mg

We examine the recent report by Prochter et al. that gamma-ray burst (GRB) sight lines have a much higher incidence of strong Mg II absorption than quasar sight lines. We give further evidence that this observed difference is not due to intervening dust extinction that would cause selection effects in the quasar sample. Instead, we propose that the difference is due to the different beam sizes of GRBs and quasars — that quasar beam sizes are, on average, about twice that of GRBs, and that the intervening Mg II systems are of a similar size, i.e. . 10 cm. We also discuss some observational predictions of our proposed model. Most notably, in some cases the intervening Mg II absorbers in GRB spectra should be seen evolving, and quasars with smaller sizes should show an increased rate of Mg II absorbers. Subject headings: gamma-rays: bursts — quasars: absorption lines