A Determination ofH0with the CLASS Gravitational Lens B1608+656. I. Time Delay Measurements with the VLA

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Abstract
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 period from 1996 October to 1997 May. The 64 epochs of observation have an average spacing of 3.6 days. The light curves of the four images of the background source show that the flux density of the background source has varied at the ~5% level. We measure time delays in the system based on common features that are seen in all four light curves. The three independent time delays in the system are found to be ΔtBA = 31 ± 7 days, ΔtBC = 36 ± 7 days, and ΔtBD = 76 days at 95% confidence. The uncertainties on the time delays are determined by Monte Carlo simulations that use fake light curves that have the characteristics of the observed light curves. This is the first gravitational lens system for which three independent time delays have been measured. A companion paper presents a mass model for the lensing galaxy that correctly reproduces the observed image positions, flux density ratios, and time delay ratios. The last condition is crucial for determining H0 with a four-image lens. We combine the time delays with the model to obtain a value for the Hubble constant of H0 = 59 km s-1 Mpc-1 at 95% confidence (statistical) for (ΩM,ΩΛ) = (1,0). In addition, there is an estimated systematic uncertainty of ±15 km s-1 Mpc-1 from uncertainties in modeling the radial mass profiles of the lensing galaxies. The value of H0 presented in this paper is comparable to recent measurements of H0 from the gravitational lenses 0957+561, PG 1115+080, B0218+357, and PKS 1830-211.
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