1999AJ....118...14B

The lensed double QSO 0957+561 has a well-measured time delay and hence is useful for a global determination of H₀. Uncertainty in the mass distribution of the lens is the largest source of uncertainty in the derived H₀. We investigate the range of H₀ produced by a set of lens models intended to mimic the full range of astrophysically plausible mass distributions, using as constraints the numerous multiply imaged sources that have been detected. We obtain the first adequate fit to all the observations only if we include effects from the galaxy cluster beyond a constant local magnification and shear. Both the lens galaxy and the surrounding cluster must depart from circular symmetry as well. Lens models that are consistent with observations to 95% confidence level (CL) indicate H₀=104⁺³¹_–23(1-κ{d1}_30") km.s^–1.Mpc^–1. Previous weak-lensing measurements constrain the mean mass density within 30" of G1 to be κ{d1}_30"=0.26±0.16 (95% CL), implying H₀=77⁺²⁹_–24 km.s^–1.Mpc^–1 (95% CL). The best-fitting models span the range 65-80 km.s^–1.Mpc^–1. Further observations will shrink the confidence interval for both the mass model and κ{d1}_30". The range of H₀ allowed by the full gamut of our lens models is substantially larger than that implied by limiting consideration to simple power-law density profiles. We therefore caution against using simple isothermal or power-law mass models to derive H₀ from other time-delay systems. High signal-to-noise ratio imaging of multiple or extended lensed features will greatly reduce the H₀ uncertainties when fitting complex models to time-delay lenses.