1994A&A...284..125D

The modulated light in the Intermediate Polar FO Aqr at the three periods P_spin=20.9min, P_orb=4.85h and P_beat=22.5min is studied in different spectral ranges to derive information on their nature. In this system the accretion geometry, with or without an accretion disc, is still a matter of debate (Hellier 1991; Norton et al. 1992). The different orbital behaviour of phase coherence between the spin and beat pulses in the X-rays (Norton et al. 1992) and in the optical/IR regions cannot be easily accounted for by only a discless dominated geometry where the accretion flow is switching from one pole to the other each half of the beat period. We therefore propose an accretion scenario where a non-axisymmetric disc is present. In such a non-standard accretion disc an azimuthal structure provides not only the source of variable mass transfer to the white dwarf, but also a reprocessing site which is mainly viewing the X-ray emission from the lower pole. Our spectral analysis shows that reprocessing is also occurring at the surface of the secondary star. The spin pulsation in the optical and IR continua can be explained by the so-called "accretion curtain" model (Rosen et al. 1988) though an additional reprocessing component at the spin period cannot be excluded. In contrast to the X-rays, the beat optical/IR modulation is not intrinsic. Reprocessing at the surface of the secondary star and at the thickened part of the disc can also account for the orbital modulation in the UV, optical and IR regions.