From Gaia Science Alerts Working Group
DNe are cataclysmic variable stars consisting of a close binary star system in which one of the components is a white dwarf, which accretes matter from its companion. They are similar to classical novae in that the white dwarf is involved in periodic outbursts, but the mechanisms are different: classical novae result from the fusion and detonation of accreted hydrogen, while current theory suggests that dwarf novae result from instability in the accretion disk, when gas in the disk reaches a critical temperature that causes a change in viscosity, resulting in a collapse onto the white dwarf that releases large amounts of gravitational potential energy.
- They repeat on irregular intervals of a few weeks to years.
- Amplitude up to 6 mag, with A = 0.8 + 1.667log(Δtrepeat)
- typical DN have two different types of outbursts: more and less energetic.
- Classes: U Gem, Z Cam, SU UMa
- in the baseline can show small variation with periods in range 0.05-0.5 days.
- often accompanied by X-ray emission
- spectrum of a system at minimum is continuous, with broad H and He emission lines
- spectrum at maximum: the lines almost disappear or become shallow absorption lines
Example of a dwarf nova GK Perseii (Evans et al. 2009) : detection of an outburst led to Swift, UV and X-rays follow up. It revealed unusual outburst in the optical but typical outburst in X, challenging existing disc outburst models.