Long-term activity of cataclysmic variables

Abstract

We show examples of the long-term optical activity of various types of cataclysmic variables. Episodic events are the common properties of this activity which often consists of the discrete levels of brightness and/or discrete phenomena (e.g. outbursts, low state episodes). The length of the recurrence time T C of the outbursts in dwarf novae (DNe) displays complicated time changes. This suggests that the individual outbursts of a given DN depend on each other, and only a small fraction of the disk matter is accreted in a given outburst. The processes in the cool disk (between the outbursts) play a big role in DNe. The low states in novalikes and polars often occur in clusters. The state transitions sometimes occur in intermittently present cycles. In the interpretation, changes of the position of the active regions on the donor with respect to the L1 point by a differential rotation of the donor are suggested. The short high states, surrounded by the long low states, can be explained by isolated bursts of mass transfer from the donor. EI UMa is an example of the intermediate polars whose brightness can largely fluctuate on the timescale of days and also produce flares. Large changes of the mode of accretion (flares, shalow low states) occur in a similar brightness of EI UMa, hence in a similar mass accretion rate onto the WD. The long-term activity of propellers depends on the configuration of the magnetic field of the white dwarf; this activity of AR Sco, with the luminosity comparable to the deep and very long (at least several decades) low states of novalikes, is dominated by a strong orbital modulation of synchrotron emission.