Qualitative theory for lensed QSOs

We show that some characteristics of multiply imaged QSO systems are very model independent and can be deduced accurately by simply scrutinizing the relative positions of images and galaxy/lens center. These include the time-ordering of the images, the orientation of the lens potential, and the rough morphology of any ring. Other features can differ considerably between specific models; H-0 is an example. Surprisingly, properties inherited from a circularly symmetric lens system are model dependent, whereas features that arise from the breaking of circular symmetry are model independent. We first develop these results from some abstract geometric ideas, then illustrate them for some well-known systems (the quadruples Q2237+030, H1413+117, HST 14113+5211, PG 1115+080, MG 0414+0534, B1608+656, B1422+231, and RX J0911+0551 and the 10-image system B1933+507), and finally remark on two systems (B1359+154 and PMN J0134-0931) where the lens properties are more complex. We also introduce a Java applet that produces simple lens systems and helps further illustrate the concepts.