Soft X-ray transientSoft X-ray transients (often abbreviated to SXT) are also known as X-ray novae. Typical SXT:s are usually very faint, or even unobservable, in X-rays and their apparent magnitude in the optical wavelengths is about 20. 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 SXT 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.
SXT are quite rare, about 100 systems are known. SXT are a class of low-mass X-ray binaries. Typical SXT contains a K-type subgiant or dwarf that is transferring mass to a black hole through an accretion disk. In some cases the compact object is a neutron star, but black holes are more common. During "quiescence" mass is accumulating to the disk, and during outburst most of 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 luminostiy rises and outburst begins. The outer disk is further heated by intense radiation from the inner accretion disk. A similar runaway heating mechanism is operating in dwarf novae.