A determination of H-0 with the class gravitational lens B1608+656. I. Time delay measurements with the VLA

We present the results of a program to monitor the four-image gravitational lens B1608 + 656 with the VLA. The system was observed over a 7 month peri od from 1996 October to 1997 May. The 64 epochs of observation have an aver age spacing of 3.6 days. The light curves of the four images of the backgro und source show that the flux density of the background source has varied a t the similar to 5% level. We measure time delays in the system based on co mmon features that are seen in all four light curves. The three independent time delays in the system are found to be Delta t(BA) = 31 +/- 7 days, At- BC = 36 +/- 7 days, and Delta t(BD) = 76(-10)(+9) days at 95% confidence. T he uncertainties on the time delays are determined by Monte Carlo simulatio ns that use faire light curves that have the characteristics of the observe d light curves. This is the first gravitational lens system for which three independent time delays have been measured. A companion paper presents a m ass model for the lensing galaxy that correctly reproduces the observed ima ge positions, flux density ratios, and time delay ratios. The last conditio n is crucial for determining H-0 with a four-image lens. We combine the tim e delays with the model to obtain a value for the Hubble constant of H-0 = 59(-7)(+8) km s(-1) Mpc(-1) at 95% confidence (statistical) for (Omega(M), Omega(Lambda)) = (1,0). In addition, there is an estimated systematic uncer tainty of +/-15 km s(-1) Mpc(-1) from uncertainties in modeling the radial mass profiles of the lensing galaxies. The value of H-0 presented in this p aper is comparable to recent measurements of H-0 from the gravitational len ses 0957 + 561, PG 1115 + 080, B0218 + 357, and PKS 1830-211.