Vlbi Polarisation Images of the Gravitational Lens B0218+357

We present preliminary polarisation VLBI maps of the gravitational lens system B0218+357, made at 3 different frequencies. 1. Summary We have made VLBI polarisation observations of the gravitational lens system Reduction of the data was made using standard procedures in the AIPS software package. Preliminary maps are presented in Figure 1. The " core " and " knot " components , seen by Patnaik et al (1995) at 15 GHz, appear at all 3 frequencies in both A and B images. The core (right) is highly polarised-consistent with the high degree of polarisation variability shown by this source (Biggs et al, 1999). Both the A and B image paths are known to suffer high Faraday rotation, with a differential RM of 980±10 rad m −2 between the images (Patnaik et al, these proceedings). Our observations are not suitable for a direct determination of RMs as they are not simultaneous, and the polarisation angle can vary on the same timescale as the image relative delay (Biggs et al, 1999). However, the effect of differential rotation between the A and B image paths is apparent in the increase with wavelength of the difference between the core PAs of A and B. Indeed, the parallel PAs of the A and B cores at 43 GHz (where Faraday rotation is negligible) nicely demonstrates a basic property of gravitational lensing-that the PA of polarisation is unchanged by the action of the lens, even though source structural position angles may be changed in the images. Acknowledgments. The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.