Soft X-ray transient
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Soft X-ray transients (SXTs), also known as X-ray novae and black hole X-ray transients, are composed of a compact object (most commonly a black hole but sometimes a neutron star) and some type of "normal", low-mass star (i.e. a star with a mass of some fraction of the Sun's mass).[1][2] These objects show dramatic changes in their X-ray emission, probably produced by variable transfer of mass from the normal star to the compact object, a process called accretion. In effect the compact object "gobbles up" the normal star, and the X-ray emission can provide the best view of how this process occurs.[3] The "soft" name arises because in many cases there is strong soft (i.e. low-energy) X-ray emission from an accretion disk close to the compact object, although there are exceptions which are quite hard.[4]
Soft X-ray transients Cen X-4 and Aql X-1 were discovered by Hakucho, Japan's first X-ray astronomy satellite to be X-ray bursters.[5]
During active accretion episodes, called "outbursts", SXTs are bright (with typical luminosities above 1037 erg/s). Between these episodes, when the accretion is absent, SXTs are usually very faint, or even unobservable; this is called the "quiescent" state.
In the "outburst" state the brightness of the system increases by a factor of 100–10000 in both X-rays and optical. During outburst, a bright SXT is the brightest object in the X-ray sky, and the apparent magnitude is about 12. The SXTs have outbursts with intervals of decades or longer, as only a few systems have shown two or more outbursts. The system fades back to quiescence in a few months. During the outburst, the X-ray spectrum is "soft" or dominated by low-energy X-rays, hence the name Soft X-ray transients.
SXTs are quite rare; about 100 systems are known. SXTs are a class of
Some SXTs in the quiescent state show thermal X-ray radiation from the surface of a neutron star with typical luminosities ~(1032—1034) erg/s. In so called "quasi-persistent SXTs", whose periods of accretion and quiescence are particularly long (of the order of years), the cooling of the accretion-heated neutron-star crust can be observed in quiescence. Analyzing the quiescent thermal states of the SXTs and their crust cooling, one can test the physical properties of the superdense matter in the neutron stars.[8][9]
See also
References
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