EXOSAT X-ray observations of the eclipsing magnetic cataclysmic variable EX Hya

New X-ray observations of the eclipsing intermediate polar system EX Hya made with the EXOSAT satellite are presented. It is found that the X-ray eclipse is partial and that its depth increases with energy between about 0.5 and 6 keV. This is the opposite behaviour to that of the quasi-sinusoidal flux modulation at the 67-min rotation period of the magnetic white dwarf, whose amplitude is greatest at the lowest energies. It is also found that the fractional depth of the X-ray eclipse is a function of 67-min rolational phase, being greatest at the peak of the modulation. The X-ray eclipse has a gradual ingress and egress and an FWHM of 1.8 min. While there is no discernible narrow eclipse at energies below 2 keV, there is a broad dip in the orbital light curve before the eclipse phase. It is found that the 67-min modulation of the X-ray flux cannot be adequately represented in terms of changes in the parameters of simple spectral models. Consideration is given to the likely accretion geometry of EX Hya in order to explain the observations. It is argued that accretion in this and other such stars occurs over a much smaller fraction of the while dwarfs surface than previously suggested, in an arc-shaped region that follows approximately part of a small circle that is centred on the magnetic pole. Because of the offset between the dipole field and the rotation axis of the white dwarf, accretion will occur preferentially from one side of the disc. Absorption in this material provides a natural explanation for the modulation of the X-ray flux at the 67-min period and its characteristic energy-dependence. The partial X-ray eclipse can be understood in terms of a grazing occultation, by the companion star, of an emitting region which has a size comparable to that of the white dwarf. At the accretion rates appropriate to EX Hya the stand-off shock which marks the beginning of the X-ray emission region is likely to be at a substantial distance above the surface of the white dwarf, particularly in Ihe regions of the accretion ‘arc’ where the mass flux is least. Eclipse geometries involving partial occultation of a single polar emission region and occultation of one of two emitting poles are considered.