Super soft X-ray source

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Super Soft X-ray source ( English Super Soft X-Ray Source , SSS ) denotes an astronomical object whose electromagnetic radiation mainly in the field of soft x-ray radiation (from 0.1 to 2.5  keV is emitted). Most SSS have been detected in extragalactic systems , as the low-energy X-ray radiation is absorbed by interstellar matter within the Milky Way . Although only a few dozen sources are known in the Milky Way, their total number is extrapolated to a few thousand.

History and characteristics

The Super Soft X-Ray Sources were first described in 1991 after an analysis of ROSAT data from the Large Magellanic Cloud . The X-ray luminosity of the SSS can reach the Eddington limit with up to 10 38  ergs per second , while its X-ray spectra are extremely soft with an energy of 20 to 100 eV. This corresponds to a black body temperature of 10 5 to 10 6 Kelvin and is two orders of magnitude lower than with other X-ray binary stars .  

From the X-ray luminosity, their distance and the black body temperature, the radius of the Super Soft X-Ray Sources could be calculated as characteristic of white dwarfs . The spectra of the SSS are interpreted in such a way that a steady or cyclical hydrogen burning takes place on the surface of the white dwarfs in a layer that is optically thick for X-rays . This requires an influx of matter on the white dwarf of around 10 −7  solar masses per year, which in most cases is transferred from a companion to the compact star.

The mass of the companion star is in most cases at least as large as that of the accreting white dwarf. This property distinguishes Super Soft X-Ray Sources from the closely related X-ray binary stars and cataclysmic variables .

variability

Some super-soft X-ray sources remain in the hydrogen-burning state on the surface of the white dwarf for a long period of time. An emission nebula made of ionized matter has formed around the X-ray source CAL 87 , which would have taken 10,000 years to form at the current level of radiation.

In addition, SSS are often variable , both in the optical and in the X-ray range. These two spectral ranges are anticorrelated : when the X-ray brightness is at its maximum, the system shows a low visual brightness , and vice versa. The change to the opposite state only takes a few days. These changes take place cyclically every 100 days. The changes in brightness are associated with a change in the mass transfer rate from the companion to the white dwarf and are considered an indication of the double star nature of the SSS. As the mass transfer rate changes, so does the radius of the photosphere around the white dwarf, which means that the radiation is predominantly emitted in the extreme ultraviolet and absorbed by interstellar gas . The binary star systems have been classified as X-ray binary stars, cataclysmic variable, and symbiotic stars .

Super-soft X-ray sources in cataclysmic binary stars have been independently classified as V-Sagittae stars based on their properties in the optical spectrum. These are semi-separate systems with a massive white dwarf from 0.7 to 1.2 solar masses, with a main sequence star or subgiants around the common center of circles. The accretion rate is  very high at 10 −7 to 10 −5 solar masses per year, close to the Eddington limit. The burning of hydrogen on the surface of the white dwarf creates a stellar wind with an outflow rate of up to 10 −7  solar masses per year. This wind leads to a Kelvin-Helmholtz instability on the surface of the accretion disk , with the result that the surface layer is removed. After a while, this process absorbs all of the soft X-ray radiation and the stellar wind gains additional energy. The stellar wind hits the companion star and this leads to an erosion of its outer atmosphere. The expansion of the companion star sinks below the Roche limit and this ends the flow of matter to the white dwarf. The Super Soft X-Ray Source becomes transparent again and the cycle of on and off of the X-ray radiation, which runs approximately every 100 days, begins anew.

In addition to red dwarfs and late subgiants, early stars such as the Be stars can also transfer matter to the white dwarf. This does not happen by crossing the Roche limit in the binary star system, but by accretion of matter from the stellar wind of the early star. However, the accretion rate is very low, which is why the hydrogen-rich matter is initially accumulated on the surface of the white dwarf over years or even decades. Thereafter, the density exceeds a critical limit and the hydrogen burning ignites for a short period of a few weeks to months. Then the binary star system falls back into its resting state.

Nova outbursts

20% of all outbreaks of classical and repetitive novae go through a phase in which they can be detected as Super Soft X-Ray Sources and which can last up to 10 years. Novas are the result of the explosive ignition of hydrogen on the surface of a white dwarf and the expulsion of matter due to the release of energy. The resulting stellar wind leads to a pseudo photosphere , which reabsorbs the radiation and initially emits it again optically. Only when the released atmosphere has expanded enough and thus its density has decreased, the x-rays from the hydrogen burning can escape. The end of the super soft phase is interpreted as the end of the hydrogen burning on the white dwarf.

SSS as the forerunner of type Ia supernovae

Type Ia supernovae arise, among other things, when the mass of a white dwarf exceeds the Chandrasekhar limit of approx. 1.2 to 1.4 solar masses. The precursors cannot be novae, since they lose more matter in an outbreak than they have accreted before. With Super Soft X-Ray Sources, on the other hand, there is constant hydrogen burning on the surface of the white dwarf, whose mass increases during this process and the above-mentioned. May exceed limit mass. The prerequisite for this is a high mass transfer rate over a long period of time. This can occur with some cataclysmic variables such as dwarf novae in permanent eruption. In the case of symbiotic stars, thermal instability in the red giant can lead to a large mass transfer rate to the white dwarf.

In order to develop into a type Ia supernova, the mass transfer rate must neither be too high nor too low:

  • if the mass transfer rate exceeds a threshold value, the mass flow becomes unstable and the entire binary star system goes through a phase of a common shell , in which the common shell is ultimately repelled; what remains mostly a separate binary star system without further mass flow.
  • If, on the other hand, the mass transfer rate is too low, explosive hydrogen burning occurs in the form of a nova.

It is not clear how a binary star system can stay in this narrow band of parameters long enough to accrete a significant mass and exceed the Chandrasekhar limit mass.

In any case, at least a million years before the final explosion as a type Ia supernova, the binary star system would have to show continuous hydrogen burning on the surface of the white dwarf and thus soft X-rays should also be detectable. However, the number of observed Super Soft X-ray Sources is two orders of magnitude too low to make a significant contribution to the formation of Type Ia supernovae. However, this can be the result of an absorption of the soft X-rays in a hydrogen-rich shell around the binary star system. Even a stellar wind with a rate of 10 −11  solar masses per year can absorb so much X-ray radiation that it would no longer be possible to detect it. This stellar wind can be a direct result of the hydrogen burning or it can be matter that is not accreted by the white dwarf, as it does not flow off via the Lagrange point L1 .

The star density in globular clusters is so high that dynamic interactions result in the formation of closer binary star systems. In addition to cataclysmic variables, this also includes the X-ray binary stars, which are observed about 200 times more frequently in globular clusters than in the general galactic field. Therefore, type Ia supernovae should also show an increased frequency in the direction of globular clusters if they develop with a certain probability from Super Soft X-Ray Sources. However, several investigations in nearby galaxies could not show any correspondence between the position of Ia supernovae and a globular star cluster.

Other sources of extremely soft X-rays

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

For the two types of white dwarfs that do not occur in a binary star system (Post-AGB objects and PG1159 stars), the X-rays are thermal radiation from the recently uncovered star nucleus.

In the case of the AM and DQ Herculis stars, the X-rays are produced by the warming of the surface of the white dwarf around the magnetic poles, at which the accreted matter is abruptly slowed down ( bremsstrahlung ).

Examples

Individual evidence

  1. ^ Walter Lewin, Michael van der Klies: Compact Stellar X-ray Sources (Cambridge Astrophysics) . Cambridge University Press, Cambridge 2010, ISBN 978-0-521-15806-0 .
  2. ^ J. Trümper et al: X-ray survey of the Large Magellanic Cloud by ROSAT . In: Nature . tape 349 , 1991, pp. 579-583 , doi : 10.1038 / 349579a0 .
  3. ^ Walter HG Lewin, Jan van Paradijs, Edward PJ van den Heuvel: X-ray Binaries . Cambridge University Press, 1997, ISBN 978-0-521-59934-4 .
  4. ↑ AF Rajoelimanana et al .: Optical and X-ray properties of CAL 83: I. Quasi-periodic Optical and Supersoft Variability . In: Astrophysics. Solar and Stellar Astrophysics . 2013, arxiv : 1304.4109v1 .
  5. ↑ Remillard, RA; Rappaport, S .; Macri, LM: Ionization nebulae surrounding CAL 83 and other supersoft X-ray sources . In: The Astrophysical Journal . tape 439 , no. 2 , 1995, p. 646 .
  6. ^ C. Alcock et al .: The X-ray off-state of the supersoft source CAL 83 and its interpretation . In: Monthly Notice of the Royal Astronomical Society . tape 286 , 1997, pp. 483-486 .
  7. ^ S. Kafka et al .: QU Carinae: Supernova Ia in the making? In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1206.6798 .
  8. ↑ KL Li et al .: A LUMINOUS BE + WHITE DWARF SUPERSOFT SOURCE IN THE WING OF THE SMC: MAXI J0158-744 . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1207.5023v1 .
  9. ^ Michael F. Bode, A. Evans: Classical novae. Cambridge Univ. Press, Cambridge 2008, ISBN 978-0-521-84330-0
  10. ↑ DR van Rossum, J.-U. Ness: Expanding atmosphere models for SSS spectra of novae . In: Astronomical News . tape 331 , Volume 2, 2010, p. 175–178 , doi : 10.1002 / asna.200911321 .
  11. ^ David Branch, Mario Livio, LR Yungelson, Francesca R. Boffi, E. Baron: In Search of the Progenitors of Type IA Supernovae . In: Publications of the Astronomical Society of the Pacific . tape 107 , 1995, pp. 1019 , doi : 10.1086 / 133657 .
  12. ^ Joseph Patterson, John R. Thorstensen, Robert Fried, David R. Skillman, Lewis M. Cook and Lasse Jensen: Superhumps in Cataclysmic Binaries. XX. V751 Cygni . In: Publications of the Astronomical Society of the Pacific . tape 113 , 2001, p. 72-81 , doi : 10.1086 / 317973 .
  13. ^ S. Rappaport, R. Di Stefano, JD Smith: Formation and evolution of luminous supersoft X-ray sources . In: The Astrophysical Journal . tape 426 , 1994, pp. 692-703 , doi : 10.1086 / 174106 .
  14. ↑ Bo Wanga, Zhanwen Hana: Progenitors of type Ia supernovae . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1204.1155v1 .
  15. ↑ Mikkel Nielsen, Carsten Dominik, Gijs Nelemans, Rasmus Voss: Obscuration of Supersoft X-ray Sources by Circumbinary Material - A Way to Hide Type Ia Supernova Progenitors? In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1207.6310v1 .
  16. ^ Pearce C. Washabaugh and Joel N. Bregman: The Production Rate of SN Ia Events in Globular Clusters . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1205.0588v1 .
  17. ^ J. Greiner: Catalog of supersoft X-ray sources . In: Astrophysics. Solar and Stellar Astrophysics . 2000, doi : 10.1016 / S1384-1076 (00) 00018-X , arxiv : astro-ph / 0005238v1 .