2010ApJ...711..201S

Strong gravitational lens systems with measured time delays between the multiple images provide a method for measuring the "time-delay distance" to the lens, and thus the Hubble constant. We present a Bayesian analysis of the strong gravitational lens system B1608+656, incorporating (1) new, deep Hubble Space Telescope (HST) observations, (2) a new velocity-dispersion measurement of 260±15 km/s for the primary lens galaxy, and (3) an updated study of the lens' environment. Our analysis of the HST images takes into account the extended source surface brightness, and the dust extinction and optical emission by the interacting lens galaxies. When modeling the stellar dynamics of the primary lens galaxy, the lensing effect, and the environment of the lens, we explicitly include the total mass distribution profile logarithmic slope γ' and the external convergence κ_ext; we marginalize over these parameters, assigning well-motivated priors for them, and so turn the major systematic errors into statistical ones. The HST images provide one such prior, constraining the lens mass density profile logarithmic slope to be γ' = 2.08±0.03; a combination of numerical simulations and photometric observations of the B1608+656 field provides an estimate of the prior for κ_ext: 0.10^+0.08_–0.05. This latter distribution dominates the final uncertainty on H₀. Fixing the cosmological parameters at Ω_m= 0.3, Ω_Λ= 0.7, and w = -1 in order to compare with previous work on this system, we find H₀= 70.6^+3.1 _–3.1 km/s/Mpc. The new data provide an increase in precision of more than a factor of 2, even including the marginalization over κ_ext. Relaxing the prior probability density function for the cosmological parameters to that derived from the Wilkinson Microwave Anisotropy Probe (WMAP) five-year data set, we find that the B1608+656 data set breaks the degeneracy between Ω_m and Ω_Λat w = -1 and constrains the curvature parameter to be -0.031 < Ω_k< 0.009 (95% CL), a level of precision comparable to that afforded by the current Type Ia SNe sample. Asserting a flat spatial geometry, we find that, in combination with WMAP, H₀= 69.7^+4.9_–5.0 km/s/Mpc and w = -0.94^+0.17_–0.19(68% CL), suggesting that the observations of B1608+656 constrain w as tightly as the current Baryon Acoustic Oscillation data do.