Super Soft X-ray Source

Super Soft X -ray source and super-soft X-ray source called an astronomical object whose electromagnetic radiation is predominantly in the range of soft X-rays emitted ( from 0.1 to 2.5 keV). Most SSS have been detected in extragalactic systems, since within the Milky Way, the low-energy X-ray radiation is absorbed by interstellar matter. Although only a few dozen sources in the Milky Way are known, their total number is extrapolated to a few thousand.

History and characteristics

The supersoft X -ray sources were first described in 1991 after an analysis of ROSAT data of the Large Magellanic Cloud. The X-ray luminosity of the SSS can reach the Eddington limit with up to 1038 erg per second, while their X-ray spectra with an energy of 20 to 100 eV are extremely soft. This corresponds to a black body temperature of 105 to 106 Kelvin, and is two orders of magnitude lower than other X-ray binaries.

From the X-ray luminosity, their distance and the black body temperature of the radius of the Supersoft X - Ray Sources as could be calculated characteristic of white dwarfs. The spectra of the SSS be interpreted that takes place on the surface of white dwarfs steady or cyclic hydrogen burning in an X-ray optically thick layer. For this purpose a Materieeinstrom is needed on the white dwarf of about 10-7 solar masses per year, which is transferred in most cases from a companion to the compact star.

The mass of the companion star is equal to or even greater than that of the accreting white dwarf in most cases. This characteristic distinguishes Supersoft X - Ray Sources from the closely related X-ray binaries and cataclysmic variables.

Variability

Some Supersoft X - Ray Sources remain for a long period in a state of hydrogen burning on the surface of the white dwarf. To the X-ray source CAL 87 is an emission nebula of ionized matter has formed, its formation would have taken the current radiation level around 10,000 years. Next SSS are often variable, both in the optical and X-ray range, said two spectral regions are anti- correlated. If the X-ray brightness at maximum is the system displays a low visual brightness, and the change in the low X-ray light lasts only a few days. These changes occur cyclically in the order of 100 days. The brightness changes are associated with a change in the mass transfer rate from the companion to the white dwarf in conjunction and are regarded as indicative of the binary nature of the SSS. With the variability of the mass transfer rate also changes the radius of the photosphere to the white dwarf, so that the radiation is predominantly emitted in the extreme ultraviolet and absorpiert by interstellar gas. The binary star systems have been classified as X-ray binaries, cataclysmic variables and symbiotic stars as.

Supersoft X - Ray Sources in cataclysmic binaries were independently as V- Sagittae star classified according to their properties in the optical spectrum. It is semi- separated systems with a high-mass white dwarf of 0.7 to 1.2 solar masses orbiting a main-sequence star or subgiant around the common center of gravity. The accretion rate is very high, close to the Eddington limit of 10-7 to 10-5 solar masses per year. Due to the hydrogen burning on the surface of the white dwarf produces a stellar wind with a Abströmrate of up to 10-7 solar masses per year. This wind leads to the surface of the accretion disk to the Kelvin - Helmholtz instability with the result that the surface layer is removed. After some time, the entire soft X-ray radiation is absorbed by this process and the stellar wind is gaining extra energy. The stellar wind strikes the companion star, and this leads to an erosion of its outer atmosphere. The expansion of the companion star falls below the Roche limit and thereby the flow of matter ends to the white dwarf. The supersoft X -ray source is transparent again and about every 100 days running cycle, the on and off of the X-rays, begins again.

In addition to red dwarfs or late subgiant also early stars such as the Be -star matter can be transferred to the white dwarf. This does not happen by exceeding the Roche limit in the double star system, but by accretion of matter from the stellar wind of early star. However, the accretion rate is very low and therefore the hydrogen-rich matter is first accumulated over years to decades on the surface of the white dwarf. Then the density exceeds a critical threshold and the hydrogen burning ignites for a short period of several weeks to months. After that, the binary system falls back to its idle state.

Nova outbursts

20% of all outbreaks of classical and recurrent novae undergo a phase in which they can be detected as a supersoft X -ray sources, and can last up to 10 years. Novae are the result of an explosive ignition of hydrogen on the surface of a white dwarf and the ejection of matter due to the release of energy. The resulting stellar wind leads to a pseudo- photosphere, the reabsorbed radiation and initially radiates again in the optical. Only when the atmosphere has dropped far enough extended and thus decreased their density, the X-ray radiation of the hydrogen burning can emerge. The end of the super- soft phase is interpreted as the end of the hydrogen burning on the white dwarf.

SSS as precursors of supernovae of type Ia

Supernovae of type Ia arise, among other things, if the mass of a white dwarf exceeds the Chandrasekhar limit of about 1.2 to 1.4 solar masses. The precursor can be no novae, as they during an outbreak lose more matter than they have accreted before. In Super Soft X - Ray Sources, however, there is a constant hydrogen burning on the surface of the white dwarf, whose mass increases during this process and can exceed the mass limit. This requires a high mass transfer rate over a long period. This may cause some cataclysmic variables such as the dwarf nova occur in permanent outburst. In symbiotic stars, a thermal instability in the red giant lead to a large mass transfer rate on the white dwarf. May, to evolve into a supernova of type Ia, the mass transfer rate be neither too high nor too low. If the mass transfer rate of the mass flow is unstable and exceeds a threshold value the entire binary system through a common -shell phase, wherein at the end of the sheath is repelled. Back remains mostly a separate binary star system without additional mass flow. If the mass transfer rate is too low leads to explosive hydrogen burning in the form of novae. It is not clear how a binary star system can remain long enough in this narrow band parameters to accrete a significant mass to exceed the chandrasekharsche mass limit.

In any case, would have at least one million years before the final eruption as a supernova of type Ia the binary star system, a continuous hydrogen burning on the surface of the white dwarf show and therefore soft X-rays to be detectable. However, the number of observed supersoft X -ray sources around two orders of magnitude too small to make a significant contribution to the development of Type Ia supernovae. But this may be the result of absorption of the soft X-ray in a hydrogen-rich shell to the binary system. Already a stellar wind at a rate of 10-11 solar masses per year can absorb so much radiation that detection would not be possible. This stellar wind can be directly a consequence of hydrogen burning or matter that is not accreted by the white dwarf, since it does not flow through the Lagrangian point L1.

In globular clusters, the star density is so high that it comes through dynamic interaction to the frequent formation of close binary systems. In addition to cataclysmic variables include the X-ray binaries, which are observed with a frequency approximately 200 times higher in globular clusters than in the general galactic field. Therefore, should also supernovae of type Ia, if they develop a certain probability of Supersoft X - Ray Sources, have an increased frequency in the direction of globular clusters. Several studies have demonstrated no correspondence between the position of supernova Ia with a globular clusters in nearby galaxies.

Other sources extremely soft X-ray

In addition to the white dwarfs, on the surface there is a permanent hydrogen burning in the form of symbiotic stars, novae, cataclysmic variables and X-ray binaries, there are a number of other astronomical sources of super soft X-rays:

• White dwarfs immediately after the post-AGB phase
• Pulsating white dwarfs of class PG1159
• Active galactic nuclei
• Planetary Nebulae
• Supernova remnants
• Polar
• DQ Herculis stars

In the two types of white dwarfs that are not in a binary system occur ( post-AGB objects and PG1159 stars), it is in the X-ray radiation to thermal radiation of the exposed recently star core. In the AM Herculis stars and DQ Herculis stars the X-rays produced by the heating of the surface of the white dwarf to the magnetic poles, where the accreted matter is decelerated abruptly.