Variability inthe

We present the results of the first systematic study of correlated variability in the X-ray emission and the parsec-scale radio structure of the ‘superluminal’ quasar 3C 345. This quasar is one of a class of core-dominated flat-spectrum radio sources which are believed to emit X-rays via the synchrotrons self-Compton process. By studying its variability in X-rays, we can test this hypothesis for 3C 345 by modeling the expected inverseCompton flux from the parsec-scale jet, using parameters derived from multifrequency VLB1 imaging, Since the predicted X-ray flux is very sensitive to the physical parameters of the compact radio-emitting regions, a study of variability, with quasisimultaneous X-ray and VLBI data, is of particular interest because it is less sensitive to assumptions in the adopted model. The soft X-ray flux density of 3C 345 was observed by the ROSAT PSPC instrument at five epochs during 1990 93, during which time its flux density varied by a factor of two, but with no change in spectral index. The X-ray points closely track the highfrequency radio flux light curve. Using a series of VLBI images, we followed the time evolution of spectral shapes and angular sizes of the nucleus and the strongest jet component ‘C7’, at a distance of R 0.5 mas (2 pc) from the nucleus, and derived physical parameters for the components. We find that component C7, not the nucleus, is the dominant generator of the observed X-rays during 1992-93. For the nucleus, we applied