THE 6 CENTIMETER LIGHT CURVES OF B0957-1994 - NEW FEATURES AND IMPLICATIONS FOR THE TIME-DELAY(561, 1979)

We report on 15 years of VLA monitoring of the gravitational lens B095 7 + 561 at 6 cm. Since our last report in 1992, there have been 32 add itional observations, in which both images have returned to their quie scent flux density levels and the A image has brightened again. We est imate the time delay from the light curves using three different techn iques: structure function analysis with chi(2) fitting, dispersion ana lysis, and a discrete correlation function. Confidence intervals for t hese time delay estimates are found using Monte Carlo techniques. With the addition of the new observations, it has become obvious that five observations from spring 1990 are not consistent with the statistical properties of the rest of the light curves, so we analyze the light c urves with those points removed, as well as the complete light curves. The three statistical techniques applied to the two versions of the d ata set result in time delay values in the range 398-461 days (or 1.09 -1.26 yr, A leading B), each with similar to 5% formal uncertainty. Th e corresponding flux ratios (B/A) are in the range 0.695-0.704. Thus, the new features in the light curve show that the time delay is less t han 500 days, in contrast with analysis of earlier versions of the rad io light curves. The large range in the time delay estimates is primar ily due to unfortunate coincidences of observing gaps with flux variat ions.