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, can be arranged in the following way, as shown in the following three panels. From top to bottom panel, the character of this activity changes from large-amplitude, isolated outbursts starting from the low baseline brightness to the dominant relatively small fluctuations in the high state.
Basic properties of the long-term activity in various kinds of CVs
The basic observable parameters of the long-term brightness variations can be summarized in the following way. Time-averaged dm/dt increases in the sequence:
Dwarf novae of U Gem type
Dwarf novae of Z Cam type
(Dwarf novae of SU UMa type are CVs with very short orbital period (mostly less than 2 hours). Their activity is similar to U Gem type, but they display two types of outburst - normal outbursts and superoutbursts.)
- Dwarf novae of U Gem type – rare outbursts
- Typical duration of outburst: days to weeks
- Typical recurrence time (cycle-length) of outbursts: weeks to years
- Typical amplitude of outburst: 2-7 mag
- Dwarf novae of SU UMa type
- Typical duration of a normal outburst: days
- Typical recurrence time (cycle-length) of normal outbursts: weeks to years
- Typical amplitude of a normal outburst: 2-3 mag
- Typical duration of a superoutburst: two weeks
- Typical recurrence time (cycle-length) of superoutbursts: weeks to years
- Typical amplitude of a superoutburst: 3-5 mag
- Dwarf novae of Z Cam type
- Typical duration of outburst: days to 1 week
- Typical recurrence time (cycle-length) of outbursts: weeks
- Typical amplitude of outburst: 2-3 mag
- Novalike and VY Scl type systems
- Typical timescale of fluctuations: days to weeks
- Typical amplitude of fluctuations: 0.5-1 mag
- Typical amplitude of high/low state transitions: 1-4 mag
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.