Investigation of the Possible Third Image and Mass Models of the Gravitational Lens PMN J1632−0033

We present multi-frequency VLBA observations of PMN J1632-0033, one of thefew gravitationally lensed quasars suspected of having a central ``odd'' image.The central component has a different spectral index than the two bright quasarimages. Therefore, either the central component is not a third image, and isinstead the active nucleus of the lens galaxy, or else it is a third imagewhose spectrum is inverted by free-free absorption in the lens galaxy. Ineither case, we have more constraints on mass models than are usually availablefor a two-image lens, especially when combined with the observed orientationsof 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 externalshear field: beta=2.05^{+0.23}_{-0.10}, where rho(r) \propto r^{-beta}. If thecentral component is a third image, a hypothesis which can be tested withfuture high-frequency observations, then the density distribution is onlyslightly shallower than isothermal: beta=1.91 +/- 0.02. We also derive limitson the size of a constant-density core, and the break radius and exponent of aninner density cusp.Comment: Accepted for publication in ApJ (24 pp., 6 figs.