Using Quadruple Lenses to Probe the Structure of the Lensing Galaxy

We show here that quadruple lenses can be useful laboratories to probewhether the potential of the lensing galaxy is purely elliptical or whether anadditional distortion is present in the deflector plane. For this test we onlyhave to know the relative image positions of the quadruple lens system and the(relative) center of light position of the lensing galaxy. Furthermore weintroduce new methods which easily allow us to determine the location (rotationangle relative to the image positions) of the major axis of the lensing galaxy.In due course we can determine the parity of the four images as well. We applythese methods to the 8 currently known quadruple lenses and find that in thecase of MG 0414+0534, CLASS 1608+656 and HST 12531-2914 it is impossible toaccommodate the relative image positions and the galaxy position with anyelliptical potential whereas the other five cases can be described very wellwith a simple elliptical potential. This method will have important impacts for$\chi^2$-fits and the reconstruction of galaxy models for quadruple lenses.Comment: 10 pages, 2 Postscript figures, uses AASTeX v4.0 macro