Structure and Magnetic Fields in the Precessing Jet System SS 433 I. Multi-Frequency Imaging from 1998

Abstract

The Very Large Array has been used at five frequencies to study the structure and linear polarization of SS433 on scales as small as ~0.1" ~ 500 AU. Each jet consists of a sharp, curving ridge-line at the leading edge, plus significant trailing off-jet emission, showing that they are enveloped by diffuse relativistic plasma. No kinematic model with constant jet speed fits our images on all scales, but they are consistent with variations in jet speed of around 10% around the optical value. Our images show continuous jets with bright components occurring simultaneously in the two jets roughly every 35 days. When corrected for projection effects and Doppler boosting, the intensities of the two jets are intrinsically very similar. Fractional linear polarization up to 20% is present along the ridge-lines, while the core is essentially unpolarized. The rotation measures are consistent with a foreground screen with RM ~ +110 radians per meter squared, plus a larger, asymmetrical contribution close to the core. The apparent magnetic fields in the jets are roughly aligned with the ridge-lines in most but not all of each jet. The jet is more highly polarized between the components than in the components themselves, suggesting that the mechanism that creates them compresses a tangled part of the magnetic field into a partially-ordered transverse layer. The off-jet emission is remarkably highly polarized, with m ~ 50% in places, suggesting large-scale order of the magnetic field surrounding the jets. This polarized signal may confuse the determination of magnetic field orientations in the jets themselves. However, the images are consistent with a jet magnetic field that is everywhere parallel to the helices.