Global 8.4‐GHz VLBI observations of JVAS B0218+357

In this paper we present new observations of the gravitational lens system JVAS B0218+357 made with a global very long baseline interferometry (VLBI) network at a frequency of 8.4 GHz. Our maps have an rms noise of 30 μJy beam - 1 and with these we have been able to image much of the extended structure of the radio jet in both the A and B images at high resolution (∼1 mas). The main use of these maps will be to enable us to further constrain the lens model for the purposes of H 0 determination. We are able to identify several subcomponents common to both images with the expected parity reversal, including one which we identify as a counter-jet. We have not been successful in detecting either the core of the lensing galaxy or a third image. Using a model of the lensing galaxy we have back-projected both of the images to the source plane and find that they agree well. However, there are small, but significant, differences which we suggest may arise from multi-path scattering in the interstellar medium (ISM) of the lensing galaxy. We also find an exponent of the radial mass distribution of β 1.04, in agreement with lens modelling of published 15-GHz VLBI data. Polarization maps of each image are presented which show that the distributions of polarization across images A and B are different. We suggest that this results from Faraday rotation and associated depolarization in the lensing galaxy.