A ROBUST DETERMINATION OF THE TIME-DELAY IN 0957-CONSTANT(561A, 0957+561B AND A MEASUREMENT OF THE GLOBAL VALUE OF HUBBLE)

Continued photometric monitoring of the gravitational lens system 0957 +/- 561A, B in the g and r bands with the Apache Point Observatory (A PO) 3.5 m telescope during 1996 shows a sharp g-band event in the trai ling (B) image light curve at the precise time predicted in an earlier paper. The prediction was based on the observation of the event durin g 1995 in the leading (A) image and on a differential time delay of 41 5 days. This success confirms the so-called short delay, and the absen ce of any such feature at a delay near 540 days rejects the ''long del ay'' for this system, thus resolving a long-standing controversy. A se ries of statistical analyses of our light-curve data yield a best-fit delay of 417 +/- 3 days (95% confidence interval) and demonstrate that this result is quite robust against variations in the analysis techni que, data subsamples, and assumed parametric relationship of the two l ight curves. Recent improvements in the modeling of the lens system (c onsisting of a galaxy plus a galaxy cluster) allow us to derive a valu e of the global value (at z = 0.36) of Hubble's constant H-0 using Ref sdal's method, a simple and direct (single-step) distance determinatio n based on experimentally verified and securely understood physics and geometry. The result is H-0 = 64 +/- 13 km s(-1) Mpc(-1) (for Omega = 1), where this 95% confidence interval is dominantly due to remaining lens model uncertainties. However, it is reassuring, that available o bservations of the lensing mass distribution overconstrain the model a nd thus provide an internal consistency check on its validity. We argu e that this determination of the extragalactic distance scale (10% acc urate at 1 sigma) is now of comparable quality, in terms of both stati stical and systematic uncertainties, to those based on more convention al techniques. Finally, we briefly discuss tile prospects for improved H, determinations using gravitational lenses, and some other possible implications and uses of the 0957 + 561A, B light curves.