The very long baseline interferometric polarization structure of 3C 345 at =2.8 cm

Abstract

Total intensity and polarization images of the quasar 3C 345 made at 2.8 cm from global very long baseline interferometric (VLBI) observations at epochs 1984.95 and 1986.17 are presented. These images have roughly a factor of 2 better resolution than most of the VLBI polarization images of this source published to date, which have been made at 6 cm. The dominant magnetic field in the VLBI jet in components further than ∼ 1.0 mas from the core is longitudinal, as had been seen in 6-cm VLBI polarization images of this source. The magnetic fields in two components in the VLBI jet were transverse at our earlier epoch, but had become longitudinal or were rotating towards becoming longitudinal by our later epoch. In a third jet component, the magnetic field changed from having an arbitrary orientation to being longitudinal between our epochs. The behaviour for these three features is consistent with a picture in which shocks forming in the VLBI jet dominate the magnetic field structure close to the core, but not further out, where a substantial underlying longitudinal magnetic field has formed, possibly as a result of shear. In two features, the transition from transverse to longitudinal magnetic field was rather prolonged and smooth, suggesting that we are seeing something more complex than a simple superposition of two orthogonal field components.