Polarimetry and spectroscopy of RX j1141.3-6410: a single-pole AM Her system

We present polarimetric and spectroscopic observations of the ROSAT source RX J1141.3-6410, recently identified as a polar. The detection of circular polarization variations, with an amplitude of 10 per cent, over a 3.16-h pe riod confirms that the system is a polar (AM Herculis star). Supporting evi dence comes from the nature of the emission lines and their radial velocity variability. In addition, we observe continuum slope changes in the far-re d spectral region (similar to 6000-8200 Angstrom), indicative of phase depe ndent cyclotron emission. Polarimetric modelling at two wavelengths establi shes RX J1141.3-6410 as a single-pole system, with i similar to beta simila r to 70 degrees. The accretion region is extended in magnetic longitude, an d is totally self-occulted for similar to 25 per cent of the orbit. The rad ial velocity curves derived from the emission lines show a phasing with max imum blueshift occurring with Delta phi similar to 0.05 of maximum intensit y and circular polarisation. In addition, the broader component of the line s exhibit a substantial radial velocity phase shift with respect to the nar rower component, in the sense that the broad component preceeds the narrow. This can be readily understood if the narrower component is principally a result of orbital motion of the stream material and the broad component mai nly a result of streaming motion near the coupling region. The phasing of t he Ca ii near-infrared line radial velocities also supports this general pi cture.