Interstellar broadening of images in the gravitational lens PKS 1830-211

The remarkably strong radio gravitational lens PKS 1830-211 consists of a 1 '' diameter Einstein ring with two bright compact (milliarcsecond) components located on opposite sides of the ring, We have obtained 22 GHz Very Long Baseline Array (VLBA) data on this source in order to determine the intrinsic angular sizes of the compact components. Previous VLBI observations at lower frequencies indicate that the brightness temperatures of these components are significantly lower than 10(10) K (Jauncey et al.), less than is typical for compact synchrotron radio sources and less than is implied by the short timescales of flux density variations. A possible explanation is that interstellar scattering is broadening the apparent angular size of the source and thereby reducing the observed brightness temperature, Our VLBA data support this hypothesis, At 22 GHz, the measured brightness temperature is at least 10(11) K, and the deconvolved size of the core in the southwest compact component is proportional to v(-2) between 1.7 and 22 GHz. VLBI observations at still higher frequencies should be unaffected by interstellar scattering.