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). 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. 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.
17-685: SXT may refer to: Soft X-ray transient or X-ray nova soft X-ray telescope , a telescope that views soft X-rays an instrument on the Yohkoh ( SOLAR-A ) space probe an instrument on the Astrosat space probe an instrument on the Hitomi ( ASTRO-H ) space probe Saxitoxin Co-trimoxazole SXT Technology Solutions, a European telecommunications company
34-400: 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; 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
51-457: 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
68-445: A large factor, then drop back down to a quiescent state. They were initially called novae (from Latin 'new'), since ones with an outburst brightness visible to the naked eye and an invisible quiescent brightness appeared as new stars in the sky. Cataclysmic variable stars are binary stars that consist of two components; a white dwarf primary, and a mass transferring secondary. The stars are so close to each other that
85-401: Is presumed to be the white dwarf's period of rotation. There are over 1600 known CV systems. The catalog was frozen as of 1 February 2006 though more are discovered each year. Cataclysmic variables are among the classes of astronomical objects most commonly found by amateurs, since a cataclysmic variable in its outburst phase is bright enough to be detectable with very modest instruments, and
102-410: The gravity of the white dwarf distorts the secondary, and the white dwarf accretes matter from the companion. Therefore, the secondary is often referred to as the donor star , and it is usually less massive than the primary. The infalling matter, which is usually rich in hydrogen , forms in most cases an accretion disk around the white dwarf. Strong UV and X-ray emission is often detected from
119-585: The ICAO airline code for Servicios De Taxi Aéreo, see Airline codes-S the rail code for Salem Town railway station the NYSE stock ticker symbol for Sensient Technologies Corporation the sign extend operation for the PDP-11 architecture SXT the former trim model for a Dodge Grand Caravan . Topics referred to by the same term [REDACTED] This disambiguation page lists articles associated with
136-452: 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 ~(10 —10 ) erg/s. In so called "quasi-persistent SXTs", whose periods of accretion and quiescence are particularly long (of
153-564: The accretion disc, powered by the loss of gravitational potential energy from the infalling material. The shortest currently observed orbit in a hydrogen-rich system is 51 minutes in ZTF J1813+4251 . Material at the inner edge of disc falls onto the surface of the white dwarf primary. A classical nova outburst occurs when the density and temperature at the bottom of the accumulated hydrogen layer rise high enough to ignite runaway hydrogen fusion reactions, which rapidly convert
170-412: 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
187-661: The disc changes from a cool, dull mode to a hotter, brighter mode for a time, before reverting to the cool mode. Dwarf novae can recur on a timescale of days to decades. Cataclysmic variables are subdivided into several smaller groups, often named after a bright prototype star characteristic of the class. In some cases the magnetic field of the white dwarf is strong enough to disrupt the inner accretion disk or even prevent disk formation altogether. Magnetic systems often show strong and variable polarization in their optical light, and are therefore sometimes called polars ; these often exhibit small-amplitude brightness fluctuations at what
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#1732781039442204-419: 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,
221-399: The fusion explosion, but this has not yet been detected. Around six galactic novae (i.e. in our own galaxy ) are discovered each year, whilst models based on observations in other galaxies suggest that the rate of occurrence ought to be between 20 and 50; this discrepancy is due partly to obscuration by interstellar dust, and partly to a lack of observers in the southern hemisphere and to
238-474: The hydrogen layer to helium . If the accretion process continues long enough to bring the white dwarf close to the Chandrasekhar limit , the increasing interior density may ignite runaway carbon fusion and trigger a Type Ia supernova explosion, which would completely destroy the white dwarf. The accretion disc may be prone to an instability leading to dwarf nova outbursts, when the outer portion of
255-495: The only celestial objects easily confused with them are bright asteroids whose movement from night to night is clear. Verifying that an object is a cataclysmic variable is also fairly straightforward: they are usually quite blue objects, they exhibit rapid and strong variability, and they tend to have peculiar emission lines . They emit in the ultraviolet and X-ray ranges; they are expected also to emit gamma rays, from annihilation of positrons from proton-rich nuclei produced in
272-465: 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. Cataclysmic variable star In astronomy , cataclysmic variable stars ( CVs ) are stars which irregularly increase in brightness by
289-738: The title SXT . If an internal link led you here, you may wish to change the link to point directly to the intended article. Retrieved from " https://en.wikipedia.org/w/index.php?title=SXT&oldid=1139362210 " Category : Disambiguation pages Hidden categories: Short description is different from Wikidata All article disambiguation pages All disambiguation pages Soft X-ray transient 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 (with typical luminosities above 10 erg/s). Between these episodes, when
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