Milliarcsecond morphology of the twin quasar 0957 + 56A, B: the view through a gravitational lens

VLBI observations were made on 15 March 1981 at 13 cm of the images of the twin quasar 0957 + 56A,B. These observations probed the milliarcsecond (mas) structure of the twin quasar and its angular environs, with submilli-Jansky sensitivities. These observations yielded the first detailed studies of images from a gravitational lens, confirming the existence of the phenomenon on cosmic scales. The main results include the following: (1) the brightness distribution of images A and B can be related with a linear transformation; (2) any other compact radio images must be less than 1/30 the intensity of image B; (3) the extended (arcsecond) jet found in image A does not appear in image B; (4) a time delay between the two images may be apparent when the A and B cores are compared with A following B; (5) a compact, weak radio source, G, exists at the position of the center of the cD galaxy believed to be part of the lens and is likely to be its radio core; and (6) the relative positions of G, B, A have been found with statistical errors of less than a mas, with positions (G = (0,0)) of A-G = (1071,5018), and B-G = (-181,-1029) mas. The A and B images reveal the source to be a typical radio quasar, with a one-sided jet which extends to its core