The expected properties of dark lenses

We investigate the properties of multiple-image gravitational lens systems formed by dark matter halos. Both the Navarro-Frenk-White (NFW) and Moore et al. mass profiles are considered, and the effects of quadrupole perturbations to the lensing potential are taken into account. The systems produced by dark halos exhibit two generic properties that serve as powerful benchmarks for scrutinizing individual lens candidates: small flux ratios between the two brightest images in triples and detectable odd images. In contrast, most currently ambiguous quasar pairs consist of two components with flux ratios greater than or equal to3 : 1. Such systems are statistical outliers in the expected distribution of dark lenses and are therefore likely to be binary quasars.