An Extremely Curved Relativistic Jet in PKS 2136+141

We report the discovery of an extremely curved jet in the radio-loud quasarPKS2136+141. Multi-frequency Very Long Baseline Array (VLBA) images show abending jet making a turn-around of 210 degrees in the plane of the sky, whichis, to our knowledge, the largest ever observed change in the position angle ofan astrophysical jet. Images taken at six different frequencies, from 2.3 to 43GHz, reveal a spiral-like trajectory, which is likely a sign of an intrinsichelical geometry. A space-VLBI image, taken with the HALCA satellite at 5 GHzand having comparable resolution to our ground-based 15 GHz data, confirms thatthe bend is a frequency-independent structure. VLBA monitoring data at 15 GHz,covering eight years of observations, show knots in the jet clearly deviatingfrom ballistic motion, which suggests that the bending may be caused by agrowing helical Kelvin-Helmholtz normal mode. The jet appearance suggests ahelical wave at a frequency well below the "resonant" frequency of the jet,which indicates that the wave is driven by a periodic perturbation at the baseof the jet. We fit the observed structure in the source with a helical twist,and we find that a simple isothermal model with a constant wave speed andwavelength gives a good fit. The measured apparent velocities indicate somedegree of acceleration along the jet, which together with an observed change inthe apparent half-opening angle of the jet allow us to estimate the changes inthe angle between the local jet direction and our line of sight. We suggestthat the jet in PKS2136+141 is distorted by a helical Kelvin-Helmholtz normalmode externally driven into the jet (e.g. by precession), and that our line ofsight falls within the opening angle of the helix cone.Comment: Accepted for publication in The Astrophysical Journal, 15 pages, uses emulateapj.cl