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double quasarThe gravitational lens, Q0957+561, where two images of a quasar, at a redshift of 1.41, are created by the gravitational effects of a galaxy at a redshift of 0.36. The two images are separated by 6″, and are observed in the radio, optical, and X-ray wavebands. It was the first bona fide gravitationally-lensed image to be discovered, by Walsh and collaborators in 1979. The spectra of the two images are almost identical, including common absorption lines from intervening matter along the line of sight. A 1.1-year time-of-flight difference has been measured between the two images.
Collins Dictionary of Astronomy © Market House Books Ltd, 2006