Dynamic confinement of jets by magnetotorsional oscillations

Many quasars and active galactic nuclei (AGN) appear in radio, optical and X-ray maps as bright nuclear sources from which emerge single or double long, thin jets. When observed with high angular resolution, these jets show evidence of structure, with bright knots separated by relatively dark regions. High percentages of polarization, sometimes more then 50 per cent, indicate the non-thermal nature of the radiation, which is well explained as the synchrotron radiation of the relativistic electrons in an ordered magnetic field.
A strong collimation of jets is probably connected with ordered magnetic fields. The mechanism of magnetic collimation first suggested by Bisnovatyi-Kogan et al. was based on the initial charge separation, which led to the creation of an oscillating electrical current, which in turn produced an azimuthal magnetic field, preventing jet expansion and disappearance. Here we consider magnetic collimation associated with the torsional oscillations of a cylinder with an elongated magnetic field. Instead of initial blobs with charge separation, we consider a cylinder with a periodically distributed initial rotation around the cylinder axis. The stabilizing azimuthal magnetic field is created by torsional oscillations, meaning that charge separation is unnecessary. An approximate simplified model is developed, and an ordinary differential equation is derived and solved numerically, making it possible to estimate quantitatively the range of parameters for which jets may be stabilized by torsional oscillations.