Intrinsic Degeneracy of Gravitational Lens Time Delays: The Case for a Simple Quadruple System in ? Cold Dark Matter Cosmology

A degeneracy in the strong lens model is shown analytically. The observed time delays and quasar image positions might not uniquely determine the concentration and the extent of the lens galaxy halo mass distribution. Simply hard wiring the Hubble constant (H-0) and the cosmology (Omega Lambda) to the standard Lambda cold dark matter cosmology values might not fully lift this degeneracy, which exists rigorously even with very accurate data. Equally good fits to the images could be found in lens mass models with either a mostly Keplerian or a at rotation curve. This degeneracy in mass models makes the task of getting reliable H-0 and Lambda from strong lenses even more daunting.