Polarimetry of the eclipsing polar RX J0929.1 - 2404

Abstract

We report polarimetric, spectropolarimetric and photometric observations of the eclipsing ROSAT cataclysmic variable RX J0929.1 - 2404, which confirm that the system is a new polar (AM Herculis system). This brings the number of eclipsing polars to nine, with RX J0929.1 - 2404 being only the third such system above the period gap. Circular polarization variations from ∼ -20 to 10 per cent are seen over the 3.39-h orbital period, with a minimum around the time of eclipse. The photopolarimetric data were modelled using arc-shaped cyclotron emission regions in a centred dipole geometry. Results imply that RX J0929.1 - 2404 is a 'two-pole' system, with one emission region partially visible at all orbital phases. Spectropolarimetry observations show some evidence for the presence of cyclotron humps in the continuum, with spacings consistent with a magnetic field strength of ∼20 MG. Photometry of the eclipses provides information on the size of the emission region, which is consistent with a hotspot on the surface of the white dwarf. The eclipse duration implies an inclination in the range 70°≲i≲78°.