First case of strong gravitational lensing by a QSO: SDSS J0013+1523, at z = 0.120

Strong gravitational lensing by a foreground lens produces multiple images of a background object. Usually the lens is a galaxy and the background object a quasar. Here the authors present the first case of strong gravitational lensing by a quasar: SDSS J0013+1523, at z = 0.120. The discovery is the result of a systematic search among 22298 SDSS quasars. Having found lines at z = 0.640 in the spectrum, the authors then used sharp Keck Adaptive Optics K-band images with laser guide stars, and unveiled two images of the z= 0.640 background object within a radius of 2 arcsec from the quasar. Lensing models can then specify the mass distribution in the quasar. This discovery opens up new directions in the study of quasars and quasar host galaxies.