Simultaneous optical polarimetry and X-ray data of the near-synchronous polar RX J2115-5840

Abstract

We present simultaneous optical polarimetry and X-ray data of the near-synchronous polar RX J2115-5840. We model the polarization data using the Stokes imaging technique of fetter ct al. We find that the data are best modelled using a relatively high binary inclination and a small angle between the magnetic and spin axes. We find that, for all spin-orbit beat phases, a significant proportion of the accretion flow is directed on to the lower hemisphere of the white dwarf, producing negative circular polarization. Only for a small fraction of the beat cycle is a proportion of the flow directed on to the upper hemisphere, However, the accretion flow never occurs near the upper magnetic pole, whatever the orientation of the magnetic poles. This indicates the presence of a non-dipole field with the field strength at the upper pole significantly higher. We find that the brightest parts of the hard X-ray emitting region and the cyclotron region are closely coincident.