caustic and critical curves, and image and source positions for model A. The fit is poor, as indicated by the final $\chi^2/DOF$ of $\sim 13$.

For the other models, we assumed an elliptically-symmetric, two-dimensional potential $\phi$ for G, as in Blandford & Kochanek (1987, hereafter BK87). The adjustable parameters were $x_G$ and $y_G$; the angular core radius $\theta_c$; the radius, $\theta_E$, of the Einstein ring for G; the ellipticity $e = 1 - b/a$, where $b/a$ is the axis ratio of the mass distribution, and the position angle $\Psi$ of the major axis of the mass distribution (on the sky, E of N). We did not adjust the power-law exponent $P=2-2\alpha$ of $\phi$, where $\alpha$ is the “hardness" of BK87 (fitting experiments showed that we cannot constrain the value of $P$ usefully).

For model B, we fixed $P=1$ and $\theta_c=0$, which corresponds to a singular isothermal profile; thus, we had 5 parameters and 8 constraints. Figure 7 shows the caustics and critical curves, as well as the image and source positions for model B. Compared to model A, model B yielded an improved $\chi^2/DOF$, $\sim 8$.

Model C was the same as model B, except that we set $e=0$, and 