What are CVs?
CVs are binary systems that consist of an normal star and a white dwarf. They are typically small – a typical binary system is roughly the size of the Earth-Moon system – with an orbital period in the range 1-10 hrs. The companion star, a more or less normal star like our Sun, loses material onto the white dwarf by accretion. How does X-ray Astronomy Fit In? There are two principal energy sources in a cataclysmic variable: accretion and nuclear fusion. Since the white dwarf is very dense, the gravitational potential energy is enormous, and it is converted into X-rays during the accretion process. The efficiency of this process is typically around 0.03% (ie the energy released in X-rays is about 0.03% of the energy which would be released by the total annihilation of the same quantity of matter). The fusion of 4 hydrogen nuclei into a He nucleus has an efficiency of 0.7%. These are much lower than accretion onto neutron stars (~10%) or black holes (up to ~40%), observed in X-ray binary
