An Excess of Mg II Absorbers in BL Lacertae Objects

Three Mg II absorbers are presented, one of which is new (z=1.340 in S5 0454+844), bringing the total number of Mg II systems in the 1 Jy radio-selected BL Lac sample to 10. Five of the 10 absorption systems are at W_λ>1 Å this is a factor of 4-5 greater than the number expected based upon quasar sight lines and is 2.5-3 σ greater than the expectation value. Interpretations of this possible excess include either that some of the Mg II absorbers might be intrinsic to the BL Lac or that there is a correlation between the presence of absorbing gas in the foreground and the nearly featureless spectra of these BL Lac objects compared to quasars. Such a correlation can be created by gravitational microlensing as suggested by Ostriker & Vietri. The similarity between the optical spectra of BL Lac objects with Mg II absorption and the spectrum of the γ-ray burst source GRB 970508 suggests that models of γ-ray bursts as microlensed AGNs should be investigated.