VLBI phase reference mapping techniques and the search for the third image of 0957+561

Abstract

This paper reviews the technique of using a phase reference in VLBI to extend the coherence and to provide accurate differential astrometry and describes the search for the third image of 0957+561. The search detected a compact component that was initially seen at 2.3 GHz and reported by Gorenstein et al. (1983. Later this compact source was also observed at 8.4 GHz and 5 GHz. Whether this source is the long sought third image or the core of the lensing galaxy has been an open question for several years. The interferometer phases on the most sensitive VLBI baseline suggest that we have observed the third image.