Twisted accretion curtains in the intermediate polar FO Aquarii

Abstract

We report on a ∼37-ks XMM–Newton observation of the intermediate polar FO Aquarii, presenting X-ray and ultraviolet data from the EPIC and OM cameras. We find that the system has changed from its previously reported state of disc-overflow accretion to one of purely disc-fed accretion. We detect the previously reported ‘notch’ feature in the X-ray spin pulse, and explain it as a partial occultation of the upper accretion pole. Maximum flux of the quasi-sinusoidal ultraviolet pulse coincides with the notch, in keeping with this idea. However, an absorption dip due to the outer accretion curtains occurs 0.27 later than the expected phase, which implies that the accretion curtains are twisted, trailing the magnetic poles. This result is the opposite of that reported in PQ Gem, where accreting field lines were found to lead the pole. We discuss how such twists relate to the accretion torques and thus the observed period changes of the white dwarfs, but find no simple connection.