Super-Quasi-Periodic Oscillations

Quasi-periodic oscillations (QPOs), oscillations of short coherence lengths, are a widely observed phenomenon in close binary systems, such as cataclysmic binaries (CVs; see Patterson 1981, ApJ 248, 1067) and X-ray binaries (XBs; a review by van der Klis 1989, Ann. Rev. Astron. Astrophys. 27, 517).

It is possible that QPOs in CVs and XBs might be of common origin. In fact, typical oscillation periods are 30 - 500 s for CVs and 10 - 200 ms for XBs, which are roughly ten - a few ten times the Keplerian time-scales at the inner edge of the disks for both cases. Several mechanisms have been proposed to account for the QPOs in CVs, including a reprocessing of the light by the oribiting blobs (Patterson 1979, ApJ 234, 978) and radial oscillation of the accretion disk (see e.g., Okuda et al. 1992, MNRAS 254, 427). However, it is still premature to conclude what structure produces such a variation and periods.

We report on the discovery of QPOs with gigantic amplitudes (~20%) found in SU UMa stars during superoutbursts -- called ``super-QPOs".

The case presented here is the 1992 superoutburst of SW UMa (Kato et al. 1992, PASJ 44, L215). Superimposed with the superhumps, one can see prominent QPOs with a mean period of 6.1 minutes and an unprecedented large amplitude of 0.2 mag.

Superhumps and super-QPOs observed during the 1992 March superoutburst of SW UMa.

Fine structure of super-QPOs. Rather complicated dip features are superimposed on nearly sinusoidal variation.

The QPOs comprise of two components: a dip component with a typical duration of one minute, and a nearly sinusoidal component. We suggest that the dip component was likely to be caused by the eclipse of the central light source by a rotating blob or by a vertical inflation of the accretion disk with a horizontal dimension of 6x10^9 cm. Since the periods of the sinusoidal component and the dip component are the same, it is natural to think that this eclipsing body is also responsible for the sinusoidal variation. Since the sinusoidal component has common characteristics with usually observed QPOs (cf. Patterson 1981, ApJ 248, 1067 for SW UMa), the sinusoidal component can therefore be interpreted as being an enhancement of normal QPOs in scale, and the source of QPOs chanced to eclipse the central object to produce the dip component.

The next superoutburst of SW UMa may be due - watch closely and see what happens!

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