An event in the light curve of 0957+561A and prediction of the 1996 image B light curve

CCD photometry of the gravitational lens system 0957+561A, B in the g and r bands was obtained on alternate nights, weather permitting, from 1994 December through 1995 May using the Double Imaging Spectrograph (DIS) on the Apache Point Observatory (APO) 3.5 m telescope. The remote observing and fast instrument change capabilities of this facility allowed accumulation of light curves sampled frequently and consistently. The Honeycutt ensemble photometry algorithm was applied to the data set and yielded typical relative photometric errors of approximately 0.01 mag. Image A exhibited a sharp drop of about 0.1 mag in late 1994 December; no other strong features were recorded in either image. This event displays none of the expected generic features of a microlensing-induced flux variation and is likely to be intrinsic to the quasar; if so, it should also be seen in the B image with the lensing differential time delay. We give the expected 1996 image B light curves based on two values of the time delay and brightness ratio which have been proposed and debated in the literature. Continued monitoring of the system in the first half of 1996 should easily detect the image B event and thus resolve the time-delay controversy.