Collimation of the radio jets in 3C 31

Abstract

Observations of the radio jets in 3C 31 using the VLA show that the rates of expansion of these jets transverse to their length decrease with increasing distance from the radio core. A rapid decrease in expansion rate in the northern jet occurs where its magnetic field appears dominated by an organized circumferential component. We use the supersonic beam model of Chan and Henriksen to show that the energy of this circumferential field need only be approx.1% of the flow kinetic energy to produce the observed changes in expansion rate by an incipient magnetic pinch. If the beams in 3C 31 are described by the Chan-Henriksen model, their circumferential field strengths cannot significantly exceed their equipartition values, or pinches much stronger than those observed would result.