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

5. Summary

We have presented data obtained with the ROSAT HRI which constitute the most sensitive X-ray observations yet of the intermediate polars YY Dra and V709 Cas. In common with some previous observations of YY Dra, the power spectrum of its X-ray lightcurve shows a signal only at a frequency corresponding to 264.7 s, which is half the spin period of the white dwarf. The X-ray spin pulse, with a period of 529.3 s, therefore exhibits a double-peaked profile in which the two peaks are similar and separated by about 0.5 in phase. The power spectrum of the X-ray lightcurve of V709 Cas shows a signal at a frequency corresponding to 312.8 s, the previously determined spin period, and also at the first two harmonics of this frequency. The harmonics enabled us to measure the spin period more accurately as  s. When folded at this period, the pulse profile of V709 Cas displays a structure that is also double-peaked, although the maxima are separated by only about 0.3 in phase, and the secondary minimum is somewhat filled-in. There is no evidence for X-ray modulation at either the orbital period or the beat period of either system, so each must be disc-fed and seen at a relatively low inclination angle.

To allay a common assumption, we have emphasised that a double-peaked pulse profile is not a unique indicator of two pole accretion, and that either single-peaked or double-peaked pulse profiles can arise when accretion occurs onto both poles of the white dwarf. Following Hellier (1996) and Allan et al. (1996), we have explained that the two possibilities are related to the spin period of the white dwarf. A short white dwarf spin period is an indicator of a relatively weak magnetic field, which in turn gives rise to relatively large footprints of the accretion curtains on the white dwarf surface. Conversely, a long white dwarf spin period indicates a somewhat stronger magnetic field, which means that the footprints of the accretion curtains are smaller. In the first case the relative optical depths across and along the accretion curtain conspire to produce a double-peaked pulse profile, whilst in the second case a single-peaked pulse profile is the result. We trust that this will lay to rest the widely accepted paradigm that single-peaked X-ray pulse profiles are the result of single pole accretion whilst double-peaked X-ray pulse profiles are the only result of two pole accretion.

There are now seven intermediate polars with short spin periods which display double-peaked pulse profiles and must therefore have weak magnetic fields: AE Aqr, DQ Her, XY Ari, V709 Cas, GK Per, YY Dra and V405 Aur. None of them exhibit X-ray beat periods, so stream-fed or disc overflow accretion does not occur in these systems. Conversely, the six systems which have shown X-ray beat periods at some time all have long spin periods and must have stronger magnetic fields: FO Aqr, TX Col, BG CMi, AO Psc, V1223 Sgr and RX J1712.6-2414. We conclude that double peaked X-ray pulse profiles are an indicator of a white dwarf with a relatively weak magnetic field, whilst X-ray beat periods are an indicator of a white dwarf with a relatively strong magnetic field.

© European Southern Observatory (ESO) 1999

Online publication: June 18, 1999
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