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Astron. Astrophys. 347, 203-211 (1999)

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1. Introduction

Intermediate polars are semi-detached interacting binaries in which a magnetic white dwarf accretes material from a Roche-lobe filling, usually late-type, main sequence companion star. The accretion flow from the secondary proceeds towards the white dwarf either through an accretion disc, an accretion stream, or some combination of both (known as disc overflow accretion), until it reaches the magnetospheric radius. Here the material attaches to the magnetic field lines and follows them towards the magnetic poles of the white dwarf. The infalling material that originates from an accretion disc takes the form of arc-shaped accretion curtains, standing above the white dwarf surface. At some distance from this surface, the accretion flow undergoes a strong shock, below which material settles onto the white dwarf, releasing X-rays as it cools by thermal bremsstrahlung processes. Since the magnetic axis is offset from the spin axis of the white dwarf, this gives rise to the defining characteristic of the class, namely X-ray emission pulsed at the white dwarf spin period. If any of the material accretes directly from an accretion stream, the proportion falling onto each pole of the white dwarf will vary according to the rotation phase of the white dwarf in the reference frame of the binary. Consequently, stream-fed (or disc overflow) accretion will give rise to X-ray emission that varies with the beat period, where [FORMULA]. About twenty confirmed intermediate polars are now recognized with a similar number of candidate systems having been proposed. Comprehensive reviews of various aspects of their behaviour are given by Patterson (1994), Warner (1995), Hellier (1995; 1996) and Norton (1995).

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© European Southern Observatory (ESO) 1999

Online publication: June 18, 1999