Gravitational Lensing as a Probe of Cold Dark Matter Subhalos
Figure 4
Different configurations of a four-image lens: (a) Fold, (b) Cusp, and (c) Cross. The upper row shows the caustics and position of the source (star) in the source plane. The solid line indicates the inner caustic and the dashed line the outer caustic. A source positioned inside the inner caustic produces five images; A source positioned between the inner and outer caustic produces three images, whereas a source positioned outside the outer caustic will not be multiply imaged. In the case of multiple images, one of the images is usually highly demagnified, so that only four- and two-image lens systems are observed, respectively. The lower row shows the corresponding critical lines and resulting observable images in the lens plane. The inner caustic maps on the outer critical line and vice versa. A close pair (A, B) and a close triplet (A, B, C) are produced in the fold (a) and cusp (b) configurations, respectively.
