We present the first case of strong gravitational lensing by a QSO : SDSS J0013+1523at z = 0.120 .
The discovery is the result of a systematic search for emission lines redshifted behind QSOs , among 22 298 spectra of the SDSS data release 7 .
Apart from the z = 0.120 spectral features of the foreground QSO , the spectrum of SDSS J0013+1523 also displays the [ O II ] and H \beta emission lines and the [ O III ] doublet , all at the same redshift , z = 0.640 .
Using sharp Keck adaptive optics K-band images obtained using laser guide stars , we unveil two objects within a radius of 2″ from the QSO .
Deep Keck optical spectroscopy clearly confirms one of these objects at z = 0.640 and shows traces of the [ O III ] emission line of the second object , also at z = 0.640 .
Lens modeling suggests that they represent two images of the same z = 0.640 emission-line galaxy .
Our Keck spectra also allow us to measure the redshift of an intervening galaxy at z = 0.394 , located 3.2″ away from the line of sight to the QSO .
If the z = 0.120 QSO host galaxy is modeled as a singular isothermal sphere , its mass within the Einstein radius is M _ { E } ( r < 1 h ^ { -1 } kpc ) = 2.16 \times 10 ^ { 10 } h ^ { -1 } M _ { \odot } and its velocity dispersion is \sigma _ { SIS } = 169 km s ^ { -1 } .
This is about 1- \sigma away from the velocity dispersion estimated from the width of the QSO H \beta emission line , \sigma _ { * } ( M _ { BH } ) = 124 \pm 47 km s ^ { -1 } .
Deep optical HST imaging will be necessary to constrain the total radial mass profile of the QSO host galaxy using the detailed shape of the lensed source .
This first case of a QSO acting as a strong lens on a more distant object opens new directions in the study of QSO host galaxies .