  We identify a third image in the unique quasar lens SDSS J1029+2623, the
second known quasar lens produced by a massive cluster of galaxies. The
spectrum of the third image shows similar emission and absorption features, but
has a redder continuum than the other two images which can be explained by
differential extinction or microlensing. We also identify several lensed arcs.
Our observations suggest a complicated structure of the lens cluster at z~0.6.
We argue that the three lensed images are produced by a naked cusp on the basis
of successful mass models, the distribution of cluster member galaxies, and the
shapes and locations of the lensed arcs. Lensing by a naked cusp is quite rare
among galaxy-scale lenses but is predicted to be common among large-separation
lensed quasars. Thus the discovery can be viewed as support for an important
theoretical prediction of the standard cold dark matter model.

Astrophysics