Q0957+561: Detailed models of the gravitational lens effect

Abstract

Further observations and theoretical work on Q0957+561 are presented. The galaxy responsible for causing multiple gravitational lens images of Q0957+561 has been reobserved spectroscopically, and we give a more accurate redshift z_d =0.36. In 1980 January, component B had brightened by 0.3 mag and was equal to A. We have further analyzed our CCD data on the surrounding cluster of galaxies, and we give a list of 180 objects (146 galaxies, 32 stars, and 2 QSO images) to a limiting magnitude r=24.5. The data indicate that the center of luminosity of the cluster is about 23" west of the QSO images. Detailed models of the gravitational lens effect have been computed. These models can explain all optical and radio data for the system. They predict time delays of up to 5 years between variations in the A and B images (with A preceding B). We believe the third image is rather faint and located close to the center of the lens galaxy. It is possible that the third image contributes to the observed galaxy core. We discuss the possibility that this system may be used to measure H_0. Given two time delays among the three images, such measurements may be successful if no further complications in the imaging arise.

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