Polarimetry and spectroscopy of RX J1141.3-6410: a single-pole AM Her system

Abstract

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 period confirms that the system is a polar (AM Herculis star). Supporting evidence comes from the nature of the emission lines and their radial velocity variability. In addition, we observe continuum slope changes in the far-red spectral region (similar to 6000-8200 Angstrom), indicative of phase dependent cyclotron emission. Polarimetric modelling at two wavelengths establishes RX J1141.3-6410 as a single-pole system, with i similar to beta similar to 70 degrees. The accretion region is extended in magnetic longitude, and is totally self-occulted for similar to 25 per cent of the orbit. The radial velocity curves derived from the emission lines show a phasing with maximum blueshift occurring with Delta phi similar to 0.05 of maximum intensity and circular polarisation. In addition, the broader component of the lines exhibit a substantial radial velocity phase shift with respect to the narrower 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 mainly a result of streaming motion near the coupling region. The phasing of the Ca ii near-infrared line radial velocities also supports this general picture.