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Astron. Astrophys. 354, L37-L40 (2000)

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7. Conclusions

The ROSAT HRI monitoring of a complete X-ray outburst of the transient SSS RX J0513 has shown that the transition between the X-ray on and off states occurs with a change of the soft X-ray flux by a factor of [FORMULA] within 2-4 days. In the model of an expanding and contracting white dwarf envelope this implies that the decrease of the effective radius by a factor of [FORMULA] and the increase of the effective temperature by a factor of [FORMULA] 3 occur on the same time-scale during the X-ray turn-on and vice versa during the X-ray turn-off.

The steepest intensity variations in the optical occur on a similar time-scale as the soft X-ray flux variations. This is consistent with our model that the optical variability is caused by the varying contribution of the accretion disk illumination by the expanding and contracting envelope of the white dwarf.

The existence of typical time-scales of the optical high/ X-ray off state, of the optical low/ X-ray on state, and of the transition phases, and the fairly accurate repetition of the optical lightcurve suggests that the observed variability is driven by limit-cycle behaviour. A possible self-maintained mechanism is the periodic change of the accretion disk viscosity in response to changes of the irradiation by the hot central star. In this scenario, the mass-flow rate at the surface of the white dwarf varies while the mass-transfer rate from the companion star remains constant. Our model can qualitatively explain the observed time-scales and requires no external mechanism like the episodic occurrence of star spots near the [FORMULA] point to trigger the transition between the X-ray on and off states.

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© European Southern Observatory (ESO) 2000

Online publication: January 31, 2000