The Third Image of the Large-Separation Lensed Quasar SDSS J1029+2623
  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. 
Masamune Oguri, Eran O. Ofek, Naohisa Inada, Tomoki Morokuma, Emilio   E. Falco, Christopher S. Kochanek, Issha Kayo, Tom Br...