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

Abstract

We report on 15 years of VLA monitoring of the gravitational lens B0957+561 at 6 cm. Since our last report in 1992, there have been 32 additional observations, in which both images have returned to their quiescent flux density levels and the A image has brightened again. We estimate the time delay from the light curves using three different techniques: the chi-squared analysis of Press, Rybicki, & Hewitt (1992a,b), the dispersion analysis of Pelt et al. (1994, 1996), and the locally normalized discrete correlation function of Lehár et al. (1992). Confidence intervals for these 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 curves with those points removed, as well as the complete light curves. The three statistical techniques applied to the two versions of the data set result in time delay values in the range 398 to 461 days (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, the new 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. The large range in the time delay estimates is primarily due to unfortunate coincidences of observing gaps with flux variations.