Difference between revisions of "Triggers:CVs"
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Revision as of 14:15, 8 December 2009
Cataclysmic variables (CVs) are very active objects and their long-term activity is extremely complex. No two CVs display identical activity. The current state of art explains this variety as a result of two dominant physical processes – variations of the mass transfer rate dm/dt with time (i.e. changes of the amount of matter which is being transferred from the companion star (donor) to the WD), and the thermal instability of the accretion disk. It emerges that the complicated profiles of the light curves basicly result from a combination of these two mechanisms (e.g. Warner, 1995).
The basic systematics of the long-term activity in CVs, shown in Fig.2, can be arranged in the following way:
A subclass of cataclysmic variables exhibiting occasional drops by several magnitudes at irregular intervals. The prototype of the class had shown a deep low-state in 2008, first since 1983 (Greiner et al. 2009).
- amplitude: several magnitudes
- drop time scale: 10-30 days
- low state lasts for about 100 days
- some periodicity (P about 10 days) is present
- mechanism not well understood
- some of VY Scl stars are supersoft X-ray sources during optical low-states.
- V504 Cen has period of 4.21h and went down by 6 mag in 2006 and remained in it for nearly a year.