Disparate MG II absorption statistics towards quasars and gamma-ray bursts: a possible explanation

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 propose that the discrepancy is due to the different beam sizes of GRBs and quasars, and that the intervening Mg II systems are clumpy with the dense part of each cloudlet of a similar size as the quasars, i.e. ≲10¹⁶ cm, but bigger than GRBs. We also discuss observational predictions of our proposed model. Most notably, in some cases the intervening Mg II absorbers in GRB spectra should be seen varying, and quasars with smaller sizes should show an increased rate of strong Mg II absorbers. In fact, our prediction of variable Mg II lines in the GRB spectra has been now confirmed by Hao et al., who observed intervening Fe II and Mg II lines at z=1.48 to be strongly variable in the multi-epoch spectra of z=4.05 GRB 060206.