GLOBAL VLBI OBSERVATIONS OF THE GRAVITATIONAL LENS SYSTEM 0957+561 A,B

We present lambda 18-cm global VLBI hybrid maps of the gravitational l ens system 0957 + 561 A,B. These confirm, to high precision, previous observations of core-jet structure in both the A and B images. From th ese 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 firs t time. We also find that the eigenvalue associated with magnification perpendicular to the jet (M(2)) changes appreciably from the jet to t he core, whereas no change is detected in the other eigenvalue (M(1)). Our results provide new constraints for lens models of this system. B y comparing the VLBI map of B to the transformed VLBI map of A, we con clude that black holes with mass > 3 x 10(6) h(-1) M(circle dot) form less than 10 per cent of the dark matter in the halo of the main lensi ng galaxy, G1 (where h = H-0/100 km s(-1) Mpc(-1)).