Global VLBI observations of the gravitational lens system 0957+561 A,B

Abstract

We present λ18-cm global VLBI hybrid maps of the gravitational lens system 0957+561 A,B. These confirm, to high precision, previous observations of core-jet structure in both the A and B images. From these data we have determined a magnification matrix at the jet relating the A and B images. This is in good agreement with previous results. The opposite parity of the images is reliably established for the first time. We also find that the eigenvalue associated with magnification perpendicular to the jet (M2) changes appreciably from the jet to the core, whereas no change is detected in the other eigenvalue (M1). Our results provide new constraints for lens models of this system. By comparing the VLBI map of B to the transformed VLBI map of A, we conclude that black holes with mass > 3 × 106h–1 M⊙ form less than 10 per cent of the dark matter in the halo of the main lensing galaxy, G1 (where h = H0/100 km s–1 Mpc–1).