RADIO OBSERVATIONS OF THE GAMMA-RAY QUASAR-0528- SUPERLUMINAL MOTION AND AN EXTREME SCATTERING EVENT(134 )

We report on multifrequency radio observations made with the Effelsber g 100-m telescope, the IRAM 30-m telescope and the Green Bank Interfer ometer between 1992 and 1994 of the gamma-ray quasar 0528 + 134. We pr esent a new VLBI based map of 0528 + 134 at 22 GHz with sub-mas angula r resolution observed in November 1992. At that time the source was in a phase of brightening at all of our observing frequencies above 3 GH z. The increase of brightness may be related to activity in the unreso lved core component of the quasar. The VLBI map at 22 GHz (epoch 1992. 85) shows a one-sided core jet structure of similar to 5 mas length. A new component close to the core indicates an apparent transverse velo city of beta(app) less than or equal to 20 (H-o = 100 km sec(-1) Mpc(- 1), q(o) = 0.5). A second component, seen also previously at 8.4 GHz & 22 GHz, shows apparent superluminal motion with beta(app) = 4.4 +/- 1 .7. Superluminal motion is expected since strong Doppler boosting in t his source is required in view of the gamma-ray luminosity and the var iability timescale, which violate the compactness limit. In summer 199 3 we observed a major outburst at all frequencies higher than a few GH z, which peaked a few months after a strong outburst in high energy ga mma-rays and showed the canonical time evolution of a cooling and/or e xpanding electron distribution. Our data indicate that the outburst in the EGRET range originated very close to the central object of the AG N and that a remnant of this outburst moved further outward in the jet until it became optically thin at radio frequencies after a few month s. During the flare in July 1993 we observed with the Effelsberg 100-m telescope an unusually strong decrease of the flux density by about 5 0% at 4.75 GHz and 10.55 GHz and slightly less at 2.695 GHz. This beha viour is also seen in the monitoring data at 2.25 GHz and 8.3 GHz take n with the Green Bank Interferometer (NRL-GBI). The event lasted less than three days at the higher frequencies and more than two weeks at 2 .25 GHz. For the case that this event is related to the intense radio flare some geometrical effects like a small variation of the viewing a ngle of the quasars jet orientated very close to the line of sight are considered, but found to be an unlikely explanation for the observed behaviour. Alternatively, an extreme scattering event by a small dense plasma cloud in the line of sight is able to match the observed time lag in the lightcurves if we take into account the mas-structure of th e source and different spectra of the components on the basis of their brightness in the VLBI maps. The importance of interstellar scatterin g is stressed as 0528 + 134 is seen in the direction of the dark cloud Barnard 30 located at 400 pc distance in the Orion complex and that w e note frequent fluctuations of the order of 25% at 2.3 GHz/2.695 GHz, our lowest monitoring frequencies, while the variations at higher fre quencies are more smooth. It is shown that the appearance of the extre me scattering event in the light curves of 0528 + 134 depends strongly on its VLBI structure and on the observed spectral appearance in our two frequency VLBI data. Due to these strong constraints our successfu l modelling provides the best available evidence that unusual variabil ity behaviour of AGN may be caused by extreme scattering events and th at localised (r < 1 AU) and dense (n(e) = 100-1000 cm(-3)) plasma stru ctures do exist in the interstellar medium of our Galaxy.