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 compact binaries

Compact Binaries

linkPfeil.gifCompact binaries are (close) double stars in which at least one of the components is a linkPfeil.gifcompact star, i.e. either a linkPfeil.gifwhite dwarf, a linkPfeil.gifneutron star or a linkPfeil.gifblack hole. Of particular astrophysical interest are those binaries in which one star transfers mass to its compact companion.
Among these we count the linkPfeil.giflow-mass X-ray binaries (with a neutron star or a black hole as the mass receiving component), and the linkPfeil.gifcataclysmic variables (with a white dwarf accretor). One of the main reasons, why these objects are of interest derives from the fact that linkPfeil.gifaccretion of matter onto the compact star liberates gravitational binding energy which, in the case of a neutron star or a black hole, exceeds the energy yield of nuclear fusion by about two orders of magnitude. Therefore, these objects are usually bright X-ray sources which can be seen throughout our Galaxy and, the brightest ones among them, even much beyond the Local Group. In addition, the accreted matter can undergo further nuclear processing thereby giving rise to spectacular phenomena like linkPfeilExtern.gifsupersoft X-ray sources, linkPfeil.gifnova explosions and linkPfeilExtern.gifType Ia supernovae (involving white dwarfs), and linkPfeilExtern.gifType I X-ray bursts (on neutron stars). Accretion of the transferred matter via a disc around the compact star leads to a number of highly interesting consequences: e.g. in cataclysmic variables it is responsible for the linkPfeil.gifdwarf nova outbursts and among the linkPfeil.giflow-mass X-ray binaries for the so-called linkPfeilExtern.gifsoft X-ray transients. Furthermore, in neutron star linkPfeil.giflow-mass X-ray binaries it results in the spin-up of the accreting linkPfeil.gifneutron star and the formation of linkPfeil.gifmillisecond pulsars in binary systems.

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