Soft X-ray transients are composed of some type of compact object and some type of "normal", low mass star. These objects show changing levels of low-energy, or "soft", X-ray emission, probably produced somehow by variable transfer of mass from the normal star to the compact object. 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. 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. During active accretion episodes, called "outbursts", SXTs are bright. 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 systemfades 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 low-mass X-ray binaries. A typical SXT contains a K-type subgiant or dwarf that is transferring mass to a compact object through an accretion disk. In some cases the compact object is a neutron star, but black holes are more common. The type of compact object can be determined by observation of the system after an outburst; residualthermal emission from the surface of a neutron star will be seen whereas a black hole will not show residual emission. During "quiescence" mass is accumulating to the disk, and during outburst most of the disk falls into the black hole. The outburst is triggered as the density in the accretion disk exceeds a critical value. High density increases viscosity, which results in heating of the disk. Increasing temperature ionizes the gas, increasing the viscosity, and the instability increases and propagates throughout the disk. As the instability reaches the inner accretion disk, the X-ray luminosity rises and outburst begins. The outer disk is further heated by intense radiation from the inner accretion disk. A similar runaway heating mechanism operates in dwarf novae. Some SXTs in the quiescent state show thermal X-ray radiation from the surface of a neutron star with typical luminosities ∼ erg/s. In so called "quasi-persistent SXTs", whose periods of accretion and quiescence are particularly long, 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.
