1997A&A...324..857H

We present new high resolution VLBI images of the radio structure of quasar 0153+744 (z=2.338) at λλ 13cm, 6cm, 3.6cm, and 1.3cm, and show that its jet as projected on the sky changes direction by a full 180 degrees between the core-jet component (A) and the bright secondary component (B) located only 10 milliarcseconds away from the core. Using images published for earlier epochs, we conclude that component B is stationary with respect to A and place an upper limit of 0.7c (H₀=100km/s/Mpc, q₀=0.5) on any apparent motion. No component motions are found in the jet either, albeit with a less stringent upper limit. This is in contrast to the superluminal motion typically found in flat-spectrum radio sources, of which class 0153+744 is a member. Component A is resolved at a wavelength of 1.3cm into a core-jet structure with four distinct components, whereas B shows complex structure consisting of resolved steep-spectrum emission. At lower resolution, the structure of B is very similar at 1.3cm and at 6cm. Using B as a reference position, we measure a dependence of the core peak brightness position on the observing frequency in accordance with models for inhomogeneous synchrotron components. We find the spectral index of the optically thin emission of B to be close to the one of the steep-spectrum jet components between 6cm and 3.6cm. No extended emission is found on scales larger than about 1 arcsecond in new VLA images made at 20cm in A and C configuration. We discuss a model of a precessing mildly relativistic jet for 0153+744, and show that a reasonable fit to the observed properties of the radio structure is achieved. In this model, component B is the result of jet components superposed along the line-of-sight (LOS) and enhanced Doppler boosting due to their moving close to the LOS. The new components found close to the core with VLBI at λ=1.3cm are sufficiently well-defined so that they may be studied in future VLBI experiments to detect motion, if any, which would further constrain, or perhaps falsify, the precessing beam model.