A Determination ofH0with 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 forwhich all three independent time delays have been measured. This makes thesystem an excellent candidate for a high-quality determination of H_0 atcosmological distances. However, the original measurements of the time delayshad large (12-20%) uncertainties, due to the low level of variability of thebackground source during the monitoring campaign. In this paper, we presentresults from two additional VLA monitoring campaigns. In contrast to the ~5%variations seen during the first season of monitoring, the source flux densitychanged by 25-30% in each of the subsequent two seasons. We analyzed thecombined data set from all three seasons of monitoring to improve significantlythe precision of the time delay measurements; the delays are consistent withthose found in the original measurements, but the uncertainties have decreasedby factors of two to three. We combined the delays with revised isothermal massmodels to derive a measurement of H_0. Depending on the positions of the galaxycentroids, which vary by up to 0.1 arcsec in HST images obtained with differentfilters, we obtain H_0 = 61-65 km/s/Mpc, for (Omega_M,Omega_L) = (0.3,0.7). Thevalue of H_0 decreases by 6% if (Omega_M,Omega_L) = (1.0,0.0). The formaluncertainties on H_0 due to the time delay measurements are +/- 1 (+/- 2)km/s/Mpc for the 1-sigma (2-sigma) confidence limits. Thus, the systematicuncertainties due to the lens model, which are on the order of +/- 15 km/s/Mpc,now dominate the error budget for this system. In order to improve themeasurement of H_0 with this lens, new models that incorporate the constraintsprovided by stellar dynamics and the optical/infrared Einstein ring seen in HSTimages must be developed.Comment: Accepted for publication in ApJ. 32 pages, 8 figures (3 in color