A multi-frequency VLBA polarization study of 3C 309.1

We present high dynamic range total intensity and linear polarization images of the CSS quasar 3C309.1 obtained with the VLBA at 5 and 8.4 GHz. The standard model of CSS objects is that they are embedded in a dense environment that de-polarises the source at longer wavelengths. We find little de-polarization associated with the jet. Only the core is significantly de-polarized. We also do not find the strong gradients in rotation measure across the source that are expected in the standard model. The integrated polarization properties of the source are due to a combination of the core polarization changing rapidly with frequency and a steep spectrum jet. In addition to being a CSS object, this radio source is believed to be embedded in an extremely massive cooling flow, which should generate very large rotation measures. We find rotation measures consistent with those found in ordinary quasars, a few hundred radians per meter square in the rest frame of the source. About 40 mas from the core, the jet is disrupted and appears to inflate a small radio lobe similar to that found in FRII sources but on a much smaller scale.