Polarimetry and photometry of the new AM Herculis system RE J1844-741

Abstract

The ROSAT Wide Field Camera extreme-ultraviolet (EUV) source RE J1844-741 was identified with a 16th-magnitude cataclysmic variable with a period of 90 min by O'Donoghue et al. In this paper, we present spectropolarimetry and broad-band photometry and polarimetry of RE J1844-741 that confirm it to be a new, short-period, AM Herculis system. Our modelling of the circular polarization and light intensity curves in the 3400-4800 and 5500-8400 A bands indicates the presence of two linearly extended cyclotron emission regions on the surface of the white dwarf. The main emission region is located below the orbital plane, and it is visible for about 40 per cent of the orbital period. The secondary emission region is located above the orbital plane, is visible for approximately 60 per cent of the orbital period, and is responsible for the negatively circularly polarized intensity maxima which dominate the light curve during high states of activity.