A ginga observation of fo aqr: Discless accretion and multiple

Abstract

We present the analysis of a 32-hr GINGA observation of the intermediate polar FO Aqr. The power spectrum of the X-ray light curve, constructed using the one-dimensional clean algorithm, indicates strong modulation at the binary-orbital, white-dwarf spin and beat periods. The asymmetry between the positive and negative orbital sidebands of the spin frequency implies that the beat modulation cannot be caused by an amplitude modulation of the spin pulse and must be intrinsic to the source. We further demonstrate that the beat is unlikely to be due to a reflection effect and as a result can only arise in a discless accretion geometry in which the majority of the accretion flow switches from one magnetic pole of the white dwarf to the other every half a beat cycle. This naturally produces an apparent orbital modulation of the X-ray spin pulse, which is seen clearly in these data. It is likely that an intrinsic orbital modulation is also present, possibly arising as a result of emission from the magneto-spheric impact site. We demonstrate the use of the clean algorithm applied to a GINGA time series and use it to separate out the spin-modulated components of the X-ray flux. The resulting reconstructed pulse profiles are free of contamination by any other modulation and are described in terms of multiple emission components. The sinusoidal component of the spin profile changes with energy and is mainly caused by a variation in photoelectric absorption as a function of spin phase. The energy-independent, narrow, 'notch' component is probably produced by the occupation of a small 'hotspot'. Finally, we note that the phase-varying X-ray spectrum of FO Aqr is complex, showing a large excess flux below 4 keV, over that expected in simple bremsstrahlung models, and a strong iron fluorescence line at 6.4 keV.