The gas stream initially falls
freely and becomes later funnelled by the strong magnetic field of the white
dwarf. It reaches
infall velocities of a few thousands of kilometers per second and is stopped
abruptly on the surface of the compact star. There all the gravitational
energy is released finally as radiation. Most of the radiation from HU Aqr
originates from the hot spot with a diameter of only a few hundred kilometers.
The spot gains several billion tons of stellar matter
per second, making it luminous from the infrared to the X-ray spectral regime.
Once per orbit the companion star
intervenes between the observer and the compact star causing the deep
depression in the light curve seen in the observational movie.
(animated gif © Arno)