Helical Jet in the Gravitationally Lensed Blazar PKS1830-211
  Recent radio VLBI observations of the complex gravitationally lensed system PKS1830-211 have thrown up some questions with regard to the processes occuring at the heart of the blazar source at a redshift of 2.51, which is viewed almost straight down the jet axis. This work links, by a model of a helical jet tracked by ballistically ejected plasmons from a precessing nozzle, observations on the scale of tens of microarcseconds to those on the scale of milliarcseconds. An observed jet precession period of 1.08 years is inferred from the model, translating to an intrinsic period of 30.8 years for a source at redshift 2.51 and an assumed jet bulk velocity of 0.99 c. This fits well with the picture of an active galactic nucleus hosting a binary black hole system at its centre, with the jet emitted by one member of the system, and the precession as being due to its orbital motion around its companion. 
S. Nair, C. Jin & M.A. Garrett