Changes in the measured image separation of the gravitational lens system PKS 1830-211

We present eight epochs of 43-GHz, dual-polarization VLBA observations of the gravitational lens system PKS 1830-211, made over fourteen weeks. A bright, compact 'core' and a faint extended 'jet' are clearly seen in maps of both lensed images at all eight epochs. The relative separation of the radio centroid of the cores (as measured on the sky) changes by up to 87 mu as between subsequent epochs. A comparison with the previous 43-GHz VLBA observations made 8 months earlier shows even larger deviations in the separation of up to 201 muas. The measured changes are most likely produced by changes in the brightness distribution of the background source, enhanced by the magnification of the lens. A relative magnification matrix that is applicable on the milliarcsecond scale has been determined by relating two vectors (the 'core-jet' separations and the offsets of the polarized and total intensity emission) in the two lensed images. The determinant of this matrix, -1.13 (+/-0.61), is in good agreement with the measured flux density ratio of the two images. The matrix predicts that the 10-mas-long jet, which is clearly seen in previous 15- and 8.4-GHz VLBA observations, should correspond to a 4-mas-long jet trailing to the south-east of the south-western image. The clear non-detection of this trailing jet is a strong evidence for substructure in the lens and may require more realistic lens models to be invoked, such as that of Nair Garrett.