The 6 Centimeter Light Curves of B0957+561, 1979–1994: New Features and Implications for the Time Delay

We report on 15 years of VLA monitoring of the gravitational lens B0957+561at 6 cm. Since our last report in 1992, there have been 32 additionalobservations, in which both images have returned to their quiescent fluxdensity levels and the A image has brightened again. We estimate the time delayfrom the light curves using three different techniques: the chi-squaredanalysis of Press, Rybicki, & Hewitt (1992a,b), the dispersion analysis of Peltet al. (1994, 1996), and the locally normalized discrete correlation functionof Leh\'ar et al. (1992). Confidence intervals for these time delay estimatesare found using Monte Carlo techniques. With the addition of the newobservations, it has become obvious that five observations from Spring 1990 arenot consistent with the statistical properties of the rest of the light curves,so we analyze the light curves with those points removed, as well as thecomplete light curves. The three statistical techniques applied to the twoversions of the data set result in time delay values in the range 398 to 461days (or 1.09 to 1.26 years, A leading B), each with ~5% formal uncertainty.The corresponding flux ratios (B/A) are in the range 0.698 to 0.704. Thus, thenew features in the light curve show that the time delay is less than 500 days,in contrast with analysis of earlier versions of the radio light curves. Thelarge range in the time delay estimates is primarily due to unfortunatecoincidences of observing gaps with flux variations.