A Determination of H0 with the CLASS Gravitational Lens B1608+656. III. A Significant Improvement in the Precision of the Time Delay Measurements

The gravitational lens CLASS B1608+656 is the only four-image lens system for which all three independent time delays have been measured. This makes the system an excellent candidate for a high-quality determination of H₀ at cosmological distances. However, the original measurements of the time delays had large (12%-20%) uncertainties, due to the low level of variability of the background source during the monitoring campaign. In this paper, we present results from two additional VLA monitoring campaigns. In contrast to the ~5% variations seen during the first season of monitoring, the source flux density changed by 25%-30% in each of the subsequent two seasons. We analyzed the combined data set from all three seasons of monitoring to improve significantly the precision of the time delay measurements; the delays are consistent with those found in the original measurements, but the uncertainties have decreased by factors of 2-3. We combined the delays with revised isothermal mass models to derive a measurement of H₀. Depending on the positions of the galaxy centroids, which vary by up to 0.1" in Hubble Space Telescope (HST) images obtained with different filters, we obtain H₀=61-65 km s^-1 Mpc^-1, for (Ω_M,Ω_Λ)=(0.3,0.7). The value of H₀ decreases by 6% if (Ω_M,Ω_Λ)=(1.0,0.0). The formal uncertainties on H₀ due to the time-delay measurements are +/-1 (+/-2) km s^-1 Mpc^-1 for the 1 σ (2 σ) confidence limits. Thus, the systematic uncertainties due to the lens model, which are on the order of +/-15 km s^-1 Mpc^-1, now dominate the error budget for this system. In order to improve the measurement of H₀ with this lens, new models that incorporate the constraints provided by stellar dynamics and the optical/infrared Einstein ring seen in HST images must be developed.