THE JETS OF QUASAR 1928+738 - SUPERLUMINAL MOTION AND LARGE-SCALE STRUCTURE

The two-sided radio structure associated with the S5-quasar 1928 + 738 (z = 0.30) has been mapped at 408 and 1662 MHz with MERLIN, at resolutions of 1.5 and 0.2 arcsec, respectively. We investigate a geometrical and kinematical description of the curved jets in terms of a precessing beam model; moderate intrinsic brightness asymmetry of the jets has to be invoked to account for the apparent brightness ratio of jet to counterjet. This comparatively low ratio seems to contradict the existence of a one-sided milliarcsecond scale jet that has been reported to display superluminal motion. The mas scale jet structure may consist of boosted filaments embedded in a broad cylindrical jet. The results of five years' monitoring of the radio structure at 22 GHz with VLBI - at the highest angular resolution obtained for this source so far - display ballistic superluminal motion along a sinusoidally curved jet ridge line. Amongst three possible interpretations for this phenomenon, namely orbital motion of the central engine in a binary black hole system - which also could be responsible for the hypothetical precession, Kelvin-Helmholtz instabilities, or a helical magnetic field structure attached to a rotating accretion disk, only the last one might provide a realistic explanation of the observed phenomenon. The apparent velocity of the components observed at 22 GHz (4.Oc) is significantly lower than that observed at 5 GHz (6.5c) further down the jet by Witzel et al. (1988).