Erratum: “A New Model of the Gravitational Lens 0957+561 and a Limit on the Hubble Constant” (ApJ, 464, 92 [1996])

We present a simple mass model for the lensing galaxy in the gravitationallylensed quasar 0957+561. The model is a generalization of the singularisothermal sphere and includes a core radius, $r_c$, and a power-law index,$\eta$, defined such that mass increases as $r^\eta$ at large radius. Weapproximate the galaxy cluster surrounding the lensing galaxy with a quadraticpotential described by its convergence $\kappa$ and shear $\gamma$. We fit themodel to a recent high resolution VLBI map of the two images of 0957+561. Weobtain a tight constraint on the radial index, $1.07 < \eta <1.18$, which meansthat the lens galaxy is nearly isothermal with increasing mass-to-light ratioout to $15 h^{-1}$ kpc. We also obtain an upper limit on the core radius, $r_c< 330 h^{-1}$ pc. We use the model to calculate the Hubble constant $H_0$ as afunction of the time delay $\Delta\tau_{BA}$ between the two images: $H_0 =\left({ 82.5^{+5.9} _{-3.0} }\right) (1 - \kappa) \left({ \Delta\tau_{BA}/1.1\,{\rm yr} }\right)^{-1}$ km/s/Mpc. Once $\Delta \tau_{BA}$ is measured, thiswill provide an upper bound on $H_0$ since $\kappa$ cannot be negative. Inaddition, the model degeneracy due to $\kappa$ can be eliminated if theone-dimensional velocity dispersion $\sigma$ of the lensing galaxy is measured.We then have $H_0 = \left({ 82.5^{+8.7} _{-5.6} }\right) (\sigma / 322\,{\rmkm/s})^2 \left({ \Delta \tau_{BA} /1.1 \,{\rm yr} }\right)^{-1}$ km/s/Mpc. Weinvestigate the effects of ellipticity in the lensing galaxy and clumpiness inthe lensing cluster and find that these cause little change in our results.