THE CHANGING X-RAY LIGHT CURVES OF THE INTERMEDIATE POLAR FO AQUARII

Comparison of five X-ray observations of the intermediate polar FO Aqr reveals that the morphology of the X-ray light curve changes consider ably with time, In particular, power spectra of the 1988 Ginga and 199 3 ASCA data reveal strong sideband and orbital variations, whereas the 1990 Ginga observation does not. This suggests that the amount of str eam-fed accretion varies with epoch, and the system was accreting pred ominantly from a disc in 1990, In contrast to other intermediate polar s, the X-ray spin-pulse profiles show significant variations between o bservations, ranging from relatively sinusoidal to sawtooth-shaped at medium energies, During the 1988 and 1990 observations a notch is visi ble at spin phase zero, due to the presence of an interpulse at phase 0.85, which is absent during the other observations. At lower energies a narrow pulse of emission is seen at spin phase 0.2, We interpret th e pulse profile from the 1990 Ginga observation using a model for accr etion from a disc on to a dipolar magnetic field, the axis of which is offset from the white dwarf centre by similar to 0.15 white dwarf rad ii. In order to account for the later occurrence of the hardness-ratio maximum in 1988 and 1993, we suggest that the accretion-rate profile changes so that accretion is favoured along the field lines which trai l the magnetic pole. This also accounts for the disappearance of the i nterpulse and notch in 1993.