Changes in the angular separation of the lensed images PKS 1830-211 NE & SW.

Abstract We present 8 epochs of 7 mm dual-polarization VLBA observations of the gravitational lens system PKS 1830-211 made over the course of 14 weeks. Clear changes in the relative positions of the cores of up to 80 μarcseconds (μas) were observed between epochs (each separated by ≈2 weeks). A comparison with previous 7 mm VLBA maps shows that the separation of the cores has changed by almost 280 μas over 12 months. This leads us to conclude that changes in the brightness distribution of the mm-VLBI core of the background source must be occurring rapidly. This is the first clear observation of significant radio source evolution in a gravitational lens system. It is also the first time that changes in source structure have been detected in a distant extra-galactic source on such short time-scales. This is partly accounted for by the magnification provided by the lens system.