A Robust Determination of the Time Delay in 0957+561A, B and a Measurement of the Global Value of Hubble's Constant

Photometric monitoring of the gravitational lens system 0957+561A, B in the g and r bands with the Apache Point Observatory (APO) 3. 5 m telescope during 1996 shows a sharp g band event in the trailing (B) image light curve at the precise time predicted from the observation of an event during 1995 in the leading (A) image with a delay of 415 days. This success confirms the "short delay, " and the lack of any feature at a delay near 540 days rejects the "long delay" for this system, resolving a long-standing controversy. A series of statistical analyses of our light curve data yield a best fit delay of 417 +/- 3 days (95% confidence interval). Recent improvements in the modeling of the lens system (consisting of a galaxy and cluster) allow us to derive a value of the global (at z = 0. 36) value of Hubble's constant H₀ using Refsdal's method, a simple and direct distance determination based on securely understood physics and geometry. The result is H₀ = 63 +/- 12 km/s/Mpc (for Omega = 1) where this 95% confidence interval is dominated by remaining lens model uncertainties.