2004MNRAS.354.1239S

We present radio images of SS 433 taken in 1987 at 1.5, 5, 8 and 15 GHz using the VLA in A configuration, and taken at four more recent epochs at 5 or 1.6 GHz using MERLIN. The extremely sensitive 5-GHz VLA image appears to be inconsistent with the ballistic kinematic model predictions. However, the inclusion of a deceleration term of 0.02c per period (162.375 d) or the use of a slower constant velocity of 0.225c gives a good agreement with the observations. This slower ejection velocity is not consistent with existing measurements of the jet velocity on subarcsecond scales. The images also show a depolarization region surrounding the radio core of SS 433, with an extent ∼0.5 arcsec at 5 GHz. This depolarization region is present even at our highest observing frequency of 15 GHz. The MERLIN images confirm the presence of depolarization at four separate observing epochs. The depolarizing medium is likely to be an ionized cloud of gas associated with the stellar wind of the normal star, although we cannot rule out thermal gas in the jet material co-existing with the relativistic electrons which emit via the synchrotron mechanism. Outside the depolarizing region the jets are highly linearly polarized (up to 20 per cent). We find a rotation measure of 119±16 rad/m2 for the extended jets, consistent with previous lower-frequency studies. After correcting for foreground Faraday rotation we find an alignment between the implied magnetic field orientation and the kinematic locus. This can be explained by the stretching of the plasma tube along the kinematic locus as a result of differential motion. We also transform the rest-frame angles defining the apparent electric field direction to the observer's frame and show that, after allowing for Lorentz contraction, projection and light travel delays, the observer also sees the electric field perpendicular to the plasma tube. The observed alignment of the jet magnetic field and direction suggests that the jet consists of a continuous plasma tube with longitudinal magnetic field (as proposed for the jets emitted by quasars) and not of an unconnected series of plasmons.