Discovery of a quadruply lensed quasar: the 'clover leaf H1413 + 117

In November 1986 we began an optical search for examples of gravitational lensing in a sample of highly luminous quasars (HLQs, M_v < -29), with the aims of improving our knowledge of the quasar luminosity function, studying the dark matter content of the Universe, and redetermining some important cosmological parameters. This survey has found one new case of lensing^1,2 and the general implications of the search have been summarized³. Here we report the discovery of a second gravitational lens system in the broad absorption line quasar H1413 + 117 (refs 4-6). Four images of comparable brightness are seen, separated by ~1 arcsec. Spectra obtained of two of the images are identical apart from the presence of sharp absorption lines in one component, which are presumably due to gas clouds along the line of sight. The unique configuration of the images, together with the fairly rare occurrence of this type of quasar, makes it incontrovertible that this is a lensed system, not a cluster of quasars, and this second discovery made by imaging bright quasars establishes the power of the method for finding systems with small separations.