HU Aqr: A sketch of the rotating binary

A normal star, the red big one in the sketch below, and a compact object, a so called white dwarf, came so close to each other, that the gravitational force of the white dwarf deforms the companion star and pulls matter off from its atmosphere. The rotation period of the binary, whose light curve was shown in the observational movie, is only 125 min. The whole system fits easily inside our sun.

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)