SIMBAD references

We report on multifrequency radio observations made with the Effelsberg 100-m telescope, the IRAM 30-m telescope and the Green Bank Interferometer between 1992 and 1994 of the γ-ray quasar 0528+134. We present a new VLBI based map of 0528+134 at 22GHz with sub-mas angular resolution observed in November 1992. At that time the source was in a phase of brightening at all of our observing frequencies above 3GHz. The increase of brightness may be related to activity in the unresolved core component of the quasar. The VLBI map at 22GHz (epoch 1992.85) shows a one-sided core jet structure of ∼5mas length. A new component close to the core indicates an apparent transverse velocity of β_app≤20 (H₀=100km/s/Mpc, q₀=0.5). A second component, seen also previously at 8.4GHz & 22GHz, shows apparent superluminal motion with β_app=4.4±1.7. Superluminal motion is expected since strong Doppler boosting in this source is required in view of the γ-ray luminosity and the variability timescale, which violate the compactness limit. In summer 1993 we observed a major outburst at all frequencies higher than a few GHz, which peaked a few months after a strong outburst in high energy γ-rays and showed the canonical time evolution of a cooling and/or expanding electron distribution. Our data indicate that the outburst in the EGRET range originated very close to the central object of the AGN and that a remnant of this outburst moved further outward in the jet until it became optically thin at radio frequencies after a few months. During the flare in July 1993 we observed with the Effelsberg 100-m telescope an unusually strong decrease of the flux density by about 50% at 4.75GHz and 10.55GHz and slightly less at 2.695GHz. This behaviour is also seen in the monitoring data at 2.25GHz and 8.3GHz taken with the Green Bank Interferometer (NRL-GBI). The event lasted less than three days at the higher frequencies and more than two weeks at 2.25GHz. For the case that this event is related to the intense radio flare some geometrical effects like a small variation of the viewing angle 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 the source and different spectra of the components on the basis of their brightness in the VLBI maps. The importance of interstellar scattering is stressed as 0528+134 is seen in the direction of the dark cloud Barnard 30 located at 400pc distance in the Orion complex and that we note frequent fluctuations of the order of 25% at 2.3GHz/2.695GHz, our lowest monitoring frequencies, while the variations at higher frequencies are more smooth. It is shown that the appearance of the extreme 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 successful modelling provides the best available evidence that unusual variability behaviour of AGN may be caused by extreme scattering events and that localised (r<1AU) and dense (n_e=100-1000cm^–3) plasma structures do exist in the interstellar medium of our Galaxy.