Investigation of the possible third image and mass models of the gravitational lens PMN J 1632 – 0033

We present multi-frequency VLBA observations of PMN J1632–0033, one of the few gravitationally lensed quasars suspected of having a central “odd” image. The central component has a different spectral index than the two bright quasar images. Therefore, either the central component is not a third image, and is instead the active nucleus of the lens galaxy, or else it is a third image whose spectrum is inverted by free-free absorption in the lens galaxy. In either case, we have more constraints on mass models than are usually available for a two-image lens, especially when combined with the observed orientations of the radio jets of the two bright quasars. If there is no third quasar image, the simplest permitted model is a singular isothermal sphere in an external shear field: β = 2.05 −0.10 (2σ), where ρ(r) ∝ r. If the central component is a third image, a hypothesis which can be tested with future high-frequency observations, then the density distribution is only slightly shallower than isothermal: β = 1.91± 0.02 (2σ). We also derive limits on the size of a constant-density core, and the break radius and exponent of an inner density cusp. Subject headings: gravitational lensing — galaxies: structure — galaxies: nuclei — quasars: individual (PMN J1632–0033) NSF Astronomy & Astrophysics Postdoctoral Fellow The Very Long Baseline Array (VLBA) and Very Large Array (VLA) are operated by the National Radio Astronomy Observatory, a facility of the National Science Foundation operated under cooperative agreement by Associated Universities for Research in Astromomy, Inc.