High Rotation Measure in the Steep Spectrum Quasar 3c 147

We present VLBA polarimetric observations of the steep-spectrum quasar 3C 147 (0538+498) made at four frequencies in the available 5-GHz band, from which we derived milliarcsecond-resolution images of the total intensity, polarization, and rotation measure distributions. The jet extending to the southwest is followed out to a distance of about 200 mas from the core. The jet shows several gentle wiggles, and turns sharply toward the north near the end of its detected length. Polarization was detected in one bright feature in the inner jet; the rotation measure of this feature (' 1300 rad m−2) agrees with the known high rotation measure of the source. The observations provide direct measurements of the rotation measure in the feature for which polarization was detected, making it possible to derotate the observed polarization vectors to infer the underlying magnetic field structure. This structure is roughly transverse to what appears to be the dominant flow direction, suggesting that this bright feature is associated with a relativistic shock.