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Description
Despite numerous and detailed studies of the jets of Active Galactic Nuclei (AGN) on pc-scales, many questions are still debated. The physical nature of the jet components is one of the most prominent unsolved problems as is the launching mechanism of jets in AGN. The Quasar 1308+326 (z=0.997) allows detailed studies of the overall properties of the jet and to derive a more physical understanding of the nature and origin of jets in general. The long-term data provided by the MOJAVE (Monitoring Of Jets in Active galactic nuclei with VLBA Experiments) survey permit tracing out the structural changes in 1308+326 presented here. The long-lived jet features in this source can be followed for about two decades. We investigate the VLBI morphology and kinematics of the jet of 1308+326 to understand the physical nature of this jet and jets in general, the role of magnetic fields, and the causal connection between jet features and the launching process. 50 VLBA observations performed at 15GHz from the MOJAVE survey have been re-modeled with Gaussian components and re-analyzed (the time covered: 1995.05-2014.07). The analysis is supplemented by multi-wavelength radio-data (UMRAO, at 4.8, 8.0, and 14.5GHz) in polarization and total intensity. We fit the apparent motion of the jet features with the help of a model of a precessing nozzle. The jet features seem to be emitted with varying viewing angles and launched into an ejection cone. Tracing the component paths yields evidence for rotational motion. Radio flux-density variability can be explained as a consequence of enhanced Doppler boosting corresponding to the motion of the jet relative to the line of sight. Based on the presented kinematics and other indicators, such as electric-vector polarization position angle (EVPA)-rotation we conclude that the jet of 1308+326 has a helical structure, i.e. the components are moving along helical trajectories, while the trajectories themselves are experiencing a precessing motion as well. A model of a precessing nozzle (Qian et al., 2014RAA....14..249Q) was applied to the data and a subset of the observed jet feature paths can be modeled successfully within this model. The data through 2012 are consistent with a swing period of 16.9 years. We discuss several scenarios to explain the observed motion phenomena, including a binary black hole model. It seems unlikely that the accretion disk around the primary black hole, that is disturbed by tidal forces of the secondary black hole, is able to launch a persistent axisymmetric jet. We conclude that we are observing a rotating helix. In particular, the observed EVPA swings can be explained by a shock moving through a straight jet pervaded by a helical magnetic field. We compare our results for 1308+326 with other astrophysical scenarios where similar, wound up filamentary structures are found. They all are related to accretion-driven processes. A helically moving or wound up object is often explained by filamentary features moving along magnetic field lines of magnetic flux tubes. It seems that a "component" is plasma tracing the magnetic field which guides the motion of the radiating radio-band plasma. Further investigations and modeling are in preparation.
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