Sub-milliarcsec-scale structure of the gravitational lens B1600+434

In the gravitational lens system B1600+434 the brighter image, A, is known to show rapid variability which is not detected in the weaker image, B (Koo pmans & de Bruyn 2000). Since correlated variability is one of the fundamen tal properties of gravitational lensing, it has been proposed that image A is microlensed by stars in the halo of the lensing galaxy (Koopmans & de Br uyn 2000). We present VLBA observations of B1600+434 at 15 GHz with a resol ution of 0.5 milliarcsec to determine the source structure at high spatial resolution. The surface brightness of the images are significantly differen t, with image A being more compact. This is in apparent contradiction with the required property of gravitational lensing that surface brightness be p reserved. Our results suggest that both the lensed images may show two-side d elongation at this resolution, a morphology which does not necessarily fa vour superluminal motion. Instead these data may suggest that image B is sc atter-broadened at the lens so that its size is larger than that of A, and hence scintillates less than image A.