Optical and X-ray observations of AO Piscium and the origin of the spin pulse in intermediate polars

Abstract

Observations of AO Psc are presented which suggest that it has a magnetic field much weaker than the AM Her stars, that it possesses an accretion disc extending inwards to approximately 8 white-dwaft radii and outwards to fill the Roche lobe, and that it accretes on to narrow arcs around the magnetic poles of the white dwarf with areas less-than-or-similar-to 0.01 of the white-dwaft surface. We exclude the alternative of an intermediate polar having a field similar to the AM Her stars, not possessing a disc, and accreting primarily through ballistic 'blobs' which strike the white dwarf over approximately 0.5 of its surface.It is argued that the spin-modulated components of the X-ray, optical-continuum and line emission originate in magnetically controlled 'accretion curtains' above the white dwarf's magnetic poles. The modulations are all caused by the variation in our view of the optically thick accretion curtain with spin phase. When the upper pole points away from the observer, we see maximum area of the accretion curtain and hence maximum flux; the material is flowing on the pole from the far side of the disc, producing blueshifted emission lines. Occultation due to the upper pole disappearing over the limb does not contribute significantly to the modulation of the X-ray light curve, implying that any disappearance is compensated for by the simultaneous appearance of the lower pole. Hence, we must see similar X-ray fluxes from both poles.Intensity dips in both the X-ray light curve and the optical emission lines are caused by material thrown out of the orbital plane, perhaps by the impact of the accretion stream with the disc, in a manner analogous to the dipping activity of many low-mass X-ray binaries. This implies a moderately high inclination of approximately 60-degrees.The different amplitudes of the optical spin-pulses from one star to another can be explained by different locations of the emission region within the accretion curtain, coupled with the effect of inclination. The spin-pulse amplitude is greatest when the system is viewed edge-on.