Why Quasar Pairs are Binary Quasars and Not Gravitational Lenses

We use simple comparisons of the optical and radio properties of the wideseparation (3'' to 10'') quasar pairs to demonstrate that they are binaryquasars rather than gravitational lenses. The most likely model is that all thepairs are binary quasars, with a one-sided 2--sigma (1--sigma) upper limit of22% (8%) on the lens fraction. Simple models for the expected enhancement ofquasar activity during galaxy mergers that are consistent with the enhancementobserved at low redshift can explain the incidence, separations, redshifts,velocity differences, and radio properties of the binary quasar population.Only a modest fraction (< 5%) of all quasar activity need be associated withgalaxy mergers to explain the binary quasars.