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 gravitationally lensed quasar 0957+561. The model is a generalization of the singular isothermal 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. We approximate the galaxy cluster surrounding the lensing galaxy with a quadratic potential described by its convergence $kappa$ and shear $gamma$. We fit the model to a recent high resolution VLBI map of the two images of 0957+561. We obtain a tight constraint on the radial index, $1.07 < eta <1.18$, which means that the lens galaxy is nearly isothermal with increasing mass-to-light ratio out 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 a function of the time delay $Delta au_{BA}$ between the two images: $H_0 = left({ 82.5^{+5.9} _{-3.0} } ight) (1 - kappa) left({ Delta au_{BA}/1.1 ,{ m yr} } ight)^{-1}$ km/s/Mpc. Once $Delta au_{BA}$ is measured, this will provide an upper bound on $H_0$ since $kappa$ cannot be negative. In addition, the model degeneracy due to $kappa$ can be eliminated if the one-dimensional velocity dispersion $sigma$ of the lensing galaxy is measured. We then have $H_0 = left({ 82.5^{+8.7} _{-5.6} } ight) (sigma / 322,{ m km/s})^2 left({ Delta au_{BA} /1.1 ,{ m yr} } ight)^{-1}$ km/s/Mpc. We investigate the effects of ellipticity in the lensing galaxy and clumpiness in the lensing cluster and find that these cause little change in our results.Comment: Submitted to ApJ. 49 pages; aaspp latex + 5 postscript figures, uuencoded gzipped tar archive. Hardcopies available by request. Also available from http://cfa-www.harvard.edu/~nagrogin/papers/0957