A VLBI Search for the Origin of Wobbling in Blazar Jets

An increasing number of blazars have been reported to show jet wobbling (i.e., non-regular rotations of the structural position angle of their innermost jets in the plane of the sky with amplitudes between 20 deg. and 50 deg., and time scales between 2 yr. and 20 yr.). The physical origin for the observed jet wobbling is still poorly understood, but as this phenomenon is triggered in the innermost regions of the jets, it must be tied to fundamental properties of the inner regions of the accretion system. Thus, jet wobbling may be an interesting potential tool for supermassive black hole, accretion and jet launching studies. As part of a joint theoretical/numerical and observational effort to characterize the observational properties and differences between these three possible scenarios we have started a long-term polarimetric phase-reference 43GHz VLBA monitoring program to observe the jet structure and the absolute motion of the jet core of four of the blazars which have shown some of the clearest evidence of large amplitude jet wobbling: NRAO 150, OJ 287, 3C273, and 3C345 . Here we present this project and we argue about its suitability for future VSOP-2 observations. 1. What is Jet Wobbling? High resolution VLBI observations are revealing an increasing number of jets in AGN (in particular in blazars) showing either regular or irregular swings of the innermost jet structural position angle in the plane of the sky (e.g. see Fig. 1, and Stirling et al. 2003; Bach et al. 2005; Lobanov & Roland 2005; Savolainen et al. 2006; Agudo et al. 2007). Time scales between 2 and 20 years and structural position-angle oscillations (projected in the plane of the sky) with amplitudes from ∼ 25◦ to ∼ 50◦ are typical for the reported cases, e.g. Fig. 2. This phenomenon, which is detected only in the innermost regions of jets in blazars (essentially within the parsec scale), will be called hereafter jet wobbling. Parsec scale AGN jet curvatures and helical-like structures observed in the inner parsecs and at larger distances from the central engine are also believed to be triggered by changes in direction at the jet ejection nozzle (e.g. Fig. 1, and Dhawan et al. 1998). 2. Jet Wobbling in NRAO 150 Recently, the discovery of an extreme case of jet wobbling in the quasar NRAO 150 has been reported through the first ultra-high-resolution VLBI set of images obtained from this source at 86 GHz (3.5 mm) and 43 GHz (7 mm) (see Fig 3