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• An Extremely Curved Relativistic Jet in PKS 2136+141,
  T. Savolaien, K. Wiik, E. Valtaoja, M. Kadler, E. Ros, M. Tornikoski, M. F. Aller, & H. D. Aller
  Astrophysical Journal, submitted
  
  • Abstract We report the discovery of an extremely curved jet in the gigahertz-peaked spectrum quasar PKS2136+141. Multi-frequency Very Long Baseline Array (VLBA) images show a bending jet making a turn-around of 210 degrees in the plane of the sky, which is, to our knowledge, the largest ever observed change in a position angle of an astrophysical jet. Images taken at six different frequencies, from 2.3 to 43 GHz, reveal a spiral-like trajectory, which is likely a sign of an intrinsic helical geometry. A space-VLBI image taken with the HALCA satellite at 5 GHz, and having comparable resolution to our ground-based 15 GHz data, confirms that the bend is a frequency-independent structure. VLBA monitoring data at 15 GHz covering eight years of observations show knots in the jet clearly deviating from ballistic motion, which suggests that the bending may be caused by a growing helical instability. We fit the observed structure in the source with a model of growing helical Kelvin-Helmholtz normal mode, and we find that only a model where the helical wavelength is constant along the jet gives a good fit. The measured apparent velocities indicate some degree of acceleration along the jet, which together with an observed change in the apparent half-opening angle of the jet allow us to estimate the changes in the angle between the local jet direction and our line of sight. We suggest that the jet in PKS2136+141 is distorted by a helical Kelvin-Helmholtz normal mode externally driven into the jet (e.g. by precession), and that our line of sight falls within the opening angle of the helix cone.

Eduardo Ros ros@mpifr-bonn.mpg.de

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