The radio wavelength time delay of gravitational lens 0957+561

The gravitational lens 0957+561 was monitored with the Very Large Array fro m 1979 to 1997. The 6 cm light-curve data from 1995 to 1997 and the 4 cm da ta from 1990 to 1997 are reported here. At 4 cm the intrinsic source variat ions occur earlier and are twice as large as the corresponding variations a t 6 cm. The VLBI core and jet components have different magnification facto rs, leading to different flux ratios for the varying and nonvarying portion s of the VLA light curves. Using both the Press, Rybicki, & Hewitt Q (PRHQ) and dispersion statistical techniques, we determined the time delay, core flux ratio, and excess nonvarying B image flux density. The fits were perfo rmed for the 4 and 6 cm light curves, both individually and jointly, and we used Gaussian Monte Carlo data to estimate 68% statistical confidence leve ls. The delay estimates from each individual wavelength were inconsistent g iven the formal uncertainties, suggesting that there are unmodeled systemat ic errors in the analysis. We roughly estimate the systematic uncertainty i n the joint result from the difference between the 6 and 4 cm results, givi ng 409 +/- 30 days for the PRHQ statistic and 397 +/- 20 days for the dispe rsion statistic. These results are consistent with the current optical time delay of 417 +/- 3 days, reconciling the long-standing difference between the optical and radio light curves and between different statistical analys es. The unmodeled systematic effects may also corrupt light curves for othe r lenses, and we caution that multiple events at multiple wavelengths may b e necessary to determine an accurate delay in any lens system. Now that con sensus has been reached regarding the time delay in the 0957+561 system, th e most pressing issue remaining for determining H-o is a full understanding of the mass distribution in the lens.