Soft X-ray transient
Encyclopedia
Soft X-ray transients are composed of some type of compact object and some type of "normal", low mass star (i.e. a star with a mass of some fraction of the Sun's mass). 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 burster
s.
Typical SXTs 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 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 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 hole
s are more common. The type of compact object can be determined by observation of the system after an outburst; residual thermal 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
.
Soft X-ray transients Cen X-4 and Aql X-1 were discovered by Hakucho
Hakucho
Hakucho was Japan's first X-ray astronomy satellite, developed by the Institute of Space and Aeronautical Science...
, Japan
Japan
Japan is an island nation in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, China, North Korea, South Korea and Russia, stretching from the Sea of Okhotsk in the north to the East China Sea and Taiwan in the south...
's first X-ray astronomy
X-ray astronomy
X-ray astronomy is an observational branch of astronomy which deals with the study of X-ray observation and detection from astronomical objects. X-radiation is absorbed by the Earth's atmosphere, so instruments to detect X-rays must be taken to high altitude by balloons, sounding rockets, and...
satellite
Satellite
In the context of spaceflight, a satellite is an object which has been placed into orbit by human endeavour. Such objects are sometimes called artificial satellites to distinguish them from natural satellites such as the Moon....
to be X-ray burster
X-ray burster
X-ray bursters are one class of X-ray binary stars exhibiting periodic and rapid increases in luminosity peaked in the X-ray regime of the electromagnetic spectrum...
s.
Typical SXTs are usually very faint, or even unobservable, in X-rays and their apparent magnitude
Apparent magnitude
The apparent magnitude of a celestial body is a measure of its brightness as seen by an observer on Earth, adjusted to the value it would have in the absence of the atmosphere...
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 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 low-mass X-ray binaries. A typical SXT contains a K-type
Stellar classification
In astronomy, stellar classification is a classification of stars based on their spectral characteristics. The spectral class of a star is a designated class of a star describing the ionization of its chromosphere, what atomic excitations are most prominent in the light, giving an objective measure...
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
Neutron star
A neutron star is a type of stellar remnant that can result from the gravitational collapse of a massive star during a Type II, Type Ib or Type Ic supernova event. Such stars are composed almost entirely of neutrons, which are subatomic particles without electrical charge and with a slightly larger...
, but black hole
Black hole
A black hole is a region of spacetime from which nothing, not even light, can escape. The theory of general relativity predicts that a sufficiently compact mass will deform spacetime to form a black hole. Around a black hole there is a mathematically defined surface called an event horizon that...
s are more common. The type of compact object can be determined by observation of the system after an outburst; residual thermal 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
Cataclysmic variable star
Cataclysmic variable stars are stars which irregularly increase in brightness by a large factor, then drop back down to a quiescent state...
.