2011A&A...529A.113Z

1156+295 is a flat-spectrum quasar that is loud at both radio and γ-ray wavelengths. Previous observations of the source revealed a radio morphology on pc to kpc scales consistent with a helical jet model. We carried out VLBA observations at centimeter and millimeter wavelengths to study the structure of the innermost jet and understand the relation between the helical structure and astrophysical processes in the central engine. The source 1156+295 was observed with the VLBA at 86, 43, and 15GHz at four epochs from 10 May 2003 to 13 March 2005. The observations were carried out in a full polarization mode. The highest resolution of the observations is 0.08 mas (∼0.5pc) at 86GHz. A core-jet structure with six jet components is identified in 1156+295. Three jet components are detected for the first time. The apparent transverse velocities of the six jet components derived from proper motion measurements range between 3.6c and 11.6c, suggesting that highly relativistic jet plasma moves in the direction close to the line of sight. The overall jet has an oscillating morphology with multiple curvatures on pc scales, which might be indicative of a helical pattern. Models of a helical jet are discussed in the context of both Kelvin-Helmholtz (K-H) instability and jet precession. The K-H instability model is in closer agreement with the observed data. The overall radio structure on scales from sub-pc to kpc appears to be fitted with a hydrodynamic model in the fundamental helical mode of Kelvin-Helmholtz (K-H) instability. This helical mode with an initial characteristic wavelength of λ₀=0.2pc is excited at the base of the jet on the scale of 0.005 pc (or 10³R_s, the typical size of the broad line region of a supermassive black hole of 4.3x10⁸M_☉). A precessing jet model can also fit the observed jet structure on scales of between 10pc and 300pc. However, with the precessing jet model, additional astrophysical processes may be required to explain the bendings of the inner jet structure (1 to 10pc) and re-collimation of the large-scale jet outflow (>300pc).