DQ Herculis variable

An Intermediate Polar (also called a DQ Herculis Star) is a type of cataclysmic variable binary star system. In most cataclysmic variables, matter from a main-sequence companion star is gravitationally stripped by a white dwarf star and forms an accretion disk around the white dwarf. In intermediate polar systems, the same general scenario applies, except that the inner disk is truncated by the magnetic field of the white dwarf. In extreme instances, the disk can be fully disrupted, although this is uncommon. In the region where the disk is truncated, the gas in the disk begins to travel along the white dwarf's magnetic field lines, forming curved sheets of luminous material called accretion curtains. Disk material passes through the curtains and then accretes onto the white dwarf near one of its magnetic poles.

The name "intermediate polar" is derived from the strength of the white dwarf's magnetic field, which is between that of non-magnetic cataclysmic variable systems and strongly magnetic systems. Non-magnetic systems exhibit full accretion disks, while strongly magnetic systems (called polars or AM Herculis systems) exhibit only accretion streams which directly impact the white dwarf's magnetosphere.

There are 26 confirmed intermediate polar systems as of 14 April 2006. This represents about 1% of the 1,830 total cataclysmic variable systems presented by Downes et al. (2006) in the Catalog of Cataclysmic Variables.

Intermediate polar systems are strong x-ray emitters. The x-rays are generated by high velocity particles from the accretion stream forming a shock as they fall onto the surface of the white dwarf star. As particles decelerate and cool before hitting the white dwarf surface, bremsstrahlung x-rays are produced and may subsequently be absorbed by gas surrounding the shock region.

...
Wikipedia