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A '''luminous supersoft X-ray source''' (SSXS, or SSS) is an [[Astronomy|astronomical]] source that emits only low energy (i.e., soft) [[X-rays]]. Soft X-rays have energies in the 0.09 to 2.5 [[keV]] range, whereas hard X-rays are in the 1-201–20 keV range.<ref name=Supersoft>{{ cite web |title=Supersoft X-Ray Sources |url=http://library.thinkquest.org/27930/supersoft.htm |url-status=dead |archiveurl=https://web.archive.org/web/20080607122110/http://library.thinkquest.org/27930/supersoft.htm |archivedate=2008-06-07 }}</ref> SSSs emit few or no photons with energies above 1 keV, and most have [[effective temperaturestemperature]] below 100 eV. This means that the radiation they emit is highly ionizing and is readily absorbed by the interstellar medium. Most SSSs within our own galaxy are hidden by interstellar absorption in the galactic disk.<ref name=White/> They are readily evident in external galaxies, with ~10 found in the Magellanic Clouds and at least 15 seen in M31.<ref name=White/>
As of early 2005, more than 100 SSSs have been reported in ~20 external galaxies, the [[Large Magellanic Cloud]] (LMC), [[Small Magellanic Cloud]] (SMC), and the [[Milky Way]] (MW).<ref name=Kahabka/> Those with luminosities below ~3 x 10<sup>38</sup> [[erg]]/s are consistent with steady [[Stellar surface fusion|nuclear burning]] in accreting [[white dwarf]]s (WD)s or post-novae.<ref name=Kahabka/> There are a few SSS with luminosities ≥10<sup>39</sup> erg/s.<ref name=Kahabka/>
Super softSupersoft X-rays are believed to be produced by steady [[nuclear fusion]] on a [[white dwarf]]'s surface of material pulled from a [[binary star|binary companion]],<ref name=SSXSmpe>{{ cite web |title=Super Soft X-ray Sources - Discovered with ROSAT |url=http://www.mpe.mpg.de/~jcg/sss/sss_high.html |author=Max Planck Institute for Extraterrestrial Physics }}</ref> the so-called close-binary supersoft source (CBSS).<ref name=Greiner/> This requires a flow of material sufficiently high to sustain the fusion. Contrast this with the [[nova]], where less flow causes the material to only fuse sporadically. Super softSupersoft X-ray sources can evolve into [[type Ia supernova]], where a sudden fusion of material destroys the white dwarf, and neutron stars, through collapse.<ref name=SSXSws>{{ cite web |title=Proceedings of the Workshop on Supersoft X-ray Sources |url=http://www.mpe.mpg.de/~jcg/sss/sssw.html |author=Max Planck Institute for Extraterrestrial Physics }}</ref>
Super softSupersoft X-ray sources were first discovered by the [[Einstein Observatory]]. Further discoveries were made by [[ROSAT]].<ref name=SSXScat>{{ cite web |title=Catalog of Supersoft X-ray Sources |url=http://www.aip.de/~jcg/sss/ssscat.html |url-status=dead |archiveurl=https://web.archive.org/web/20071128104800/http://www.aip.de/~jcg/sss/ssscat.html |archivedate=2007-11-28 }}</ref> Many different classes of objects emit supersoft X-radiation (emission dominantly below 0.5 keV).<ref name=Greiner/>
==Luminous supersoft X-ray sources==
Luminous super softsupersoft X-ray sources have a characteristic blackbody temperature of a few tens of eV (~20-10020–100 eV)<ref name=Kahabka>{{ cite journal |author=Kahabka P |title=Supersoft X-ray sources |journal=Adv. SpaceAdvances Res.in Space Research|date=Dec 2006 |volume=38 |issue=12 |pages=2836–9 |url=http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6V3S-4MBT29S-2&_user=10&_origUdi=B6V3S-3YN948T-5&_fmt=high&_coverDate=12%2F31%2F2006&_rdoc=1&_orig=article&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=3a3d0440365be046b322ab561aae9230 |doi=10.1016/j.asr.2005.10.058 |bibcode=2006AdSpR..38.2836K}}</ref> and a bolometric luminosity of ~10<sup>38</sup> erg/s (below ~ 3 x 10<sup>38</sup> erg/s).<ref name=White>{{ cite journal |display-authors=4|author=White NE|author2=Giommi P|author3=Heise J|author4=Angelini L|author5=Fantasia S |title=RX J0045.4+4154: A Recurrent Supersoft X-ray Transient in M31 |journal=Astrophys. J.The Lett.Astrophysical Journal|volume=445 |pages=L125 |url=http://lheawww.gsfc.nasa.gov/users/white/wgacat/apjl.html |doi=10.1086/187905|bibcode = 1995ApJ...445L.125W |date=1995|url-status=dead|archiveurl=https://web.archive.org/web/20090703002924/http://lheawww.gsfc.nasa.gov/users/white/wgacat/apjl.html|archivedate=2009-07-03}}</ref><ref name=Kahabka/>
Apparently, luminous SSSsSSXSs can have equivalent blackbody temperatures as low as ~15 eV and luminosities ranging from 10<sup>36</sup> to 10<sup>38</sup> erg/s.<ref name=KahabkaHeuvel>{{ cite journal |author=Kahabka P|author2=van den Heuvel EPJ |title=Luminous Supersoft X-Ray Sources |journal=Annu. Rev.Annual Astron.Review Astrophys.of Astronomy and Astrophysics|date=1997 |volume=35 |issue=1 |pages=69–100 |bibcode=1997ARA&A..35...69K |doi=10.1146/annurev.astro.35.1.69 |s2cid=70774203 |url=https://pure.uva.nl/ws/files/4067674/37187_162193y.pdf }}</ref> The numbers of luminous SSSs in the disks of ordinary spiral galaxies such as the MW and M31 are estimated to be on the order of 10<sup>3</sup>.<ref name=KahabkaHeuvel/>
==Milky Way SSXSs==
SSXSs have now been discovered in our galaxy and in globular cluster M3.<ref name=White/> MR Velorum (RX J0925.7-4758) is one of the rare MW super softsupersoft X-ray binaries.<ref name=Greiner>{{ cite journal |doi=10.1016/S1384-1076(00)00018-X |author=Greiner J |title=Catalog of supersoft X-ray sources |journal= New Astron. Astronomy|date=2000 |volume=5 |issue=3 |pages=137–41 |url=http://www.mpe.mpg.de/~jcg/sss/ssscat.html |arxiv = astro-ph/0005238 |bibcode = 2000NewA....5..137G |s2cid=52241439 }}</ref> "The source is heavily reddened by interstellar material, making it difficult to observe in the blue and ultraviolet."<ref name=Schmidtke>{{ cite journal |doi=10.1086/322155 |author=Schmidtke PC |author2=Cowley AP|author2-link=Anne Cowley |title=Synoptic Observations of the Supersoft Binary MR Velorum (RX J0925.7-4758): Determination of the Orbital Period |journal=Astron. J.The Astronomical Journal|date=Sep 2001 |volume=122 |issue= 3|pages=1569–71 |url=http://www.iop.org/EJ/article/1538-3881/122/3/1569/201190.text.html |bibcode=2001AJ....122.1569S|doi-access=free |url-access=subscription }}</ref> The period determined for MR Velorum at ~4.03 d is considerably longer than that of other supersoft systems, which is usually less than a day.<ref name=Schmidtke/>
==Close-binary supersoft source (CBSS)==
The CBSS model invokes steady [[Stellar surface fusion|nuclear burning on the surface]] of an accreting [[white dwarf]] (WD) as the generator of the prodigious super softsupersoft X-ray flux.<ref name=Greiner/> As of 1999, eight SSXSs have orbital periods between ~4 hr and 1.35 d: RX J0019.8+2156 (MW), [[RX J0439.8−6809|RX J0439.8-6809]] (MW halo near LMC), RX J0513.9-6951 (LMC), RX J0527.8-6954 (LMC), RX J0537.7-7034 (LMC), CAL 83 (LMC), CAL 87 LMC), and 1E 0035.4-7230 (SMC).<ref name=Greiner/>
==Symbiotic binary==
==Noninteracting white dwarfs==
The youngest, hottest WD, [[KPD 0005+5106]], is very close to 100,000 K, of type DO and is the first single WD recorded as an X-ray source with ROSAT.<ref name=Fleming>{{ cite journal |author=Fleming TA|author2=Werner K |author3=Barstow MA |journal=Astrophys. J.The Astrophysical Journal|date=October 1993 |volume=416 |pages=L79 |title=Detection of the First Coronal X-Ray Source about a White Dwarf |bibcode=1993ApJ...416L..79F |doi=10.1086/187075 |doi-access=free }}</ref><ref name=Werner1994>{{ cite journal |title=Spectral analysis of the hottest known helium-rich white dwarf: KPD 0005+5106 |author=Werner |journal=Astron. Astrophys.Astronomy and Astrophysics|date=1994 |volume=284 |pages=907 |bibcode = 1994A&A...284..907W }}</ref>
==Cataclysmic variables==
{{main|Cataclysmic variable star}}
"Cataclysmic variables (CVs) are close binary systems consisting of a white dwarf and a red-dwarf secondary transferring matter via the Roche lobe overflow."<ref name=Kato>{{ cite journal |author=Kato T|author2=Ishioka R|author3=Uemura M |title=Photometric Study of KR Aurigae during the High State in 2001 |journal=Publ. Astron.Publications Soc.of Jpn.the Astronomical Society of Japan|volume=54 |issue=6 |date=Dec 2002 |pages=1033–9 |url=http://pasj.asj.or.jp/v54/n6/540624/540624-frame.html |arxiv = astro-ph/0209351 |bibcode = 2002PASJ...54.1033K |doi=10.1093/pasj/54.6.1033}}</ref> Both fusion- and accretion-powered cataclysmic variables have been observed to be [[X-ray]] sources.<ref name=nasa2>{{ cite web |title=Introduction to Cataclysmic Variables (CVs) |url=http://heasarc.gsfc.nasa.gov/docs/objects/cvs/cvstext.html }}</ref> The accretion disk may be prone to [[instability]] leading to [[dwarf nova]] outbursts: a portion of the disk material falls onto the white dwarf, the cataclysmic outbursts occur when the [[density]] and [[temperature]] at the bottom of the accumulated hydrogen layer rise high enough to ignite [[nuclear fusion]] reactions, which rapidly burn the hydrogen layer to helium.
Apparently the only SSXS nonmagnetic cataclysmic variable is [[V SgeSagittae]]: bolometric luminosity of (1 - 101–10) x 10<sup>37</sup>, a binary including a blackbody (BB) accretor at T < 80 eV, and an orbital period of 0.514195 d.<ref name=Greiner/>
The accretion disk can become thermally stable in systems with high mass-transfer rates (Ṁ).<ref name=Kato/> Such systems are called nova-like (NL) stars, because they lack outbursts characteristic of dwarf novae.<ref name=Osaki>{{ cite journal |doi=10.1086/133689 |last1=Osaki |first1=Yoji |title=Dwarf-Nova Outbursts |journal=Publ. Astron.Publications Soc.of Pac.the Astronomical Society of the Pacific|date=1996 |volume=108 |pages=39 |bibcode=1996PASP..108...39O |doi-access=free }}</ref>
==VY Scl cataclysmic variables==
Among the NL stars is a small group which shows a temporary reduction or cessation of Ṁ from the secondary. These are the VY Scl-type stars or anti-dwarf novae.<ref name=Warner>{{ cite book |author=Warner B |date=1995 |title=Cataclysmic Variable Stars |publisher=Cambridge University Press |___location=Cambridge |urlbibcode=http://adsabs.harvard.edu/cgi-bin/bib_query?1995cvs..book.....W }}</ref>
===V751 Cyg===
V751 Cyg (BB, MW) is a VY Scl CV, has a bolometric luminosity of 6.5 x 10<sup>36</sup> erg/s,<ref name=Greiner/> and emits soft X-rays at quiescence.<ref name=Patterson>{{ cite journal |doi=10.1086/317973 |display-authors=4 |author=Patterson J |author2=Thorstensen JR |author3=Fried R|author4=Skillman DR |author5=Cook LM |author6=Jensen L |title=Superhumps in Cataclysmic Binaries. XX. V751 Cygni |journal=Publ. Astron.Publications Soc.of Pac.the Astronomical Society of the Pacific|date=Jan 2001 |volume=113 |issue=779 |pages=72–81 |bibcode=2001PASP..113...72P |doi-access=free }}</ref> The discovery of a weak soft X-ray source of V751 Cyg at minimum presents a challenge as this is unusual for CVs which commonly display weak hard X-ray emission at quiescence.<ref name=Patterson/>
The high luminosity (6.5 x 10<sup>36</sup> erg/s) is particularly hard to understand in the context of VY Scl stars generally, because observations suggest that the binaries become simple red dwarf + white dwarf pairs at quiescence (the disk mostly disappears).<ref name=Patterson/> "A high luminosity in soft X-rays poses an additional problem of understanding why the spectrum is of only modest excitation."<ref name=Patterson/> The ratio He II λ4686/Hβ did not exceed ~0.5 in any of the spectra recorded up to 2001, which is typical for accretion-powered CVs and does not approach the ratio of 2 commonly seen in supersoft binaries (CBSS).<ref name=Patterson/>
==Magnetic cataclysmic variables==
{{main|Polar (cataclysmic variable)}}
X-rays from magnetic cataclysmic variables are common because accretion provides a continuous supply of coronal gas.<ref name=Trimble/> A plot of number of systems vs. orbit period shows a statistically significant minimum for periods between 2 and 3 hr which can probably be understood in terms of the effects of magnetic braking when the companion star becomes completely convective and the usual dynamo (which operates at the base of the convective envelope) can no longer give the companion a magnetic wind to carry off angular momentum.<ref name=Trimble/> The rotation has been blamed on asymmetric ejection of planetary nebulae and winds<ref name=Spruit>{{ cite journal |title=Origin of the rotation rates of single white dwarfs |author=Spruit HC |date=1998 |journal=Astron. Astrophys.Astronomy and Astrophysics|volume=333 |pages=603 |arxiv = astro-ph/9802141 |bibcode = 1998A&A...333..603S }}</ref> and the fields on in situ dynamos.<ref name=Schmidt97>{{ cite journal |doi=10.1086/304746 |author=Schmidt GD |author2=Grauer AD |date=1997 |title=Upper Limits for Magnetic Fields on Pulsating White Dwarfs |journal=Astrophys. J.The Astrophysical Journal|volume=488 |issue=2 |pages=827827–830 |bibcode=1997ApJ...488..827S|doi-access=free }}</ref> Orbit and rotation periods are synchronized in strongly magnetized WDs.<ref name=Trimble/> Those with no detectable field never are synchronized.
With temperatures in the range 11,000 to 15,000 K, all the WDs with the most extreme fields are far too cool to be detectable EUV/X-ray sources, e.g., Grw +70°8247, LB 11146, SBS 1349+5434, PG 1031+234 and GD 229.<ref name=Schmidt95>{{ cite journal |doi=10.1086/175962 |author=Schmidt GD |author2=Smith PS |title=A Search for Magnetic Fields among DA White Dwarfs |journal=Astrophys. J.The Astrophysical Journal|date=1995 |volume=448 |pages=305 |bibcode=1995ApJ...448..305S|doi-access=free }}</ref>
Most highly magnetic WDs appear to be isolated objects, although G 23-4623–46 (7.4 MG) and LB 1116 (670 MG) are in unresolved binary systems.<ref name=Barstow/>
RE J0317-853 is the hottest magnetic WD at 49,250 K, with an exceptionally intense magnetic field of ~340 MG, and implied rotation period of 725.4 s.<ref name=Barstow>{{ cite journal |display-authors=4|author=Barstow MA|author2=Jordan S|author3=O'Donoghue D|author4=Burleigh MR|author5=Napiwotzki R|author6=Harrop-Allin MK |title=RE J0317-853: the hottest known highly magnetic DA white dwarf |date=1995 |journal=MNRAS Monthly Notices of the Royal Astronomical Society|volume=277 |issue=3 |pages=931–85 |bibcode=1995MNRAS.277..971B |doi = 10.1093/mnras/277.3.971 |doi-access=free }}</ref> Between 0.1 and 0.4 keV, RE J0317-853 was detectable by ROSAT, but not in the higher energy band from 0.4 to 2.4 keV.<ref name=Fleming1995>{{citation cite journal needed|journaldate=AstronAugust Astrophys2017}}<!-- |title=Someone said |author=Fleming, TA |date=(1995), }}</refAstronomy and Astrophysics, but that just doesn't exist--> RE J0317-853 is associated with a blue star 16 arcsec from LB 9802 (also a blue WD) but not physically associated.<ref name=Barstow/> A centered dipole field is not able to reproduce the observations, but an off-center dipole 664 MG at the south pole and 197 MG at the north pole does.<ref name=Barstow/>
Until recently (1995) only PG 1658+441 possessed an effective temperature > 30,000 K.<ref name=Barstow/> Its polar field strength is only 3 MG.<ref name=Barstow/>
The [[ROSAT]] Wide Field Camera (WFC) source RE J0616-649 has an ~20 MG field.<ref name=Jordan>{{cite journal
| display-authors = 4| lastlast1 = Barstow
| firstfirst1 = M. A.
| last2 = Jordan
| first2 = S.
| author5 = Napiwotzki, R.
| author6 = Harrop-Allin, M. K.
| lastname-authorlist-ampstyle = yesamp
| date = December 1995
| title = RE J0317-853: the hottest known highly magnetic DA white dwarf
| journal = Monthly Notices of the Royal Astronomical Society
| url = http://adsabs.harvard.edu/abs/1995MNRAS.277..971B
| journal = MNRAS
| volume = 277
| issue = 3
| bibcode = 1995MNRAS.277..971B
| doi = 10.1093/mnras/277.3.971
| doi-access= free
| accessdate = 8 December 2014
}}</ref>
PG 1031+234 has a surface field that spans the range from ~200 MG to nearly 1000 MG and rotates with a period of 3<sup>h</sup>24<sup>m</sup>.<ref name=Latter>{{ cite journal |doi=10.1086/165543 |author=Latter WB |author2=Schmidt GD |author3=Green RF |title=The rotationally modulated Zeeman spectrum at nearly 10 to the 9th Gauss of the white dwarf PG 1031 + 234 |journal=Astrophys. J.The Astrophysical Journal|date=1987 |volume=320 |pages=308 |bibcode=1987ApJ...320..308L}}</ref>
The magnetic fields in CVs are confined to a narrow range of strengths, with a maximum of 7080 MG for RX J1938.4-4623.<ref name=Schwope>{{ cite journal |title=Two-pole accretion in the high-field polar RXJ 1938.6-4612 |author=Schwope AD|display-authors=etal |journal=Astron. Astrophys.Astronomy and Astrophysics|date=1995 |volume=293 |pages=764 |bibcode = 1995A&A...293..764S }}</ref>
None of the single magnetic stars has been seen as of 1999 as an X-ray source, although fields are of direct relevance to the maintenance of coronae in main sequence stars.<ref name=Trimble/>
==PG 1159 stars==
PG 1159 stars are a group of very hot, often pulsating WDs for which the prototype is [[PG 1159-035|PG 1159]] dominated by carbon and oxygen in their atmospheres.<ref name=Trimble>{{ cite journal |author=Trimble V |title=White dwarfs in the 1990's1990s |journal=Bull AstronBulletin Socof Indiathe Astronomical Society of India|date=1999 |volume=27 |pages=549–66 |bibcode=1999BASI...27..549T }}</ref>
PG 1159 stars reach luminosities of ~10<sup>38</sup> erg/s but form a rather distinct class.<ref name=Dreizler>{{ citeCite journalbook |author=Dreizler S|author2=Werner K|author3=Heber U |journalchapter=Lect.PG 1159 stars and their evolutionary link to DO white dwarfs Notes Phys. |date=1995 |volume=443 |pages=160160–170 |editor=Kӧster D |editor2=Werner K |title=White Dwarfs |publisher=Springer |___location=Berlin |doi=10.1007/3-540-59157-5_199|series=Lecture Notes in Physics|isbn=978-3-540-59157-3}}</ref> RX J0122.9-7521 has been identified as a galactic PG 1159 star.<ref name=Cowley>{{ cite journal |doi=10.1086/133640 |author=Cowley AP|author-link=Anne Cowley |author2=Schmidtke PC |author3=Hutchings JB |author4=Crampton D |title=X-Ray Discovery of a Hot PG1159 Star, RX J0122.9-7521 |journal=PASPPubl. Astron. Soc. Pac.|volume=107 |pages=927 |date=1995 |bibcode=1995PASP..107..927C|doi-access=free }}</ref><ref name=Werner>{{ citeCite journalbook |display-authors=4|author=Werner K|author2=Wolff B|author3=Cowley AP|author3-link=Anne Cowley|author4=Schmidtke PC|author5=Hutchings JB|author6=Crampton D |date=1996 |title= Supersoft X-rayRay Sources |editorchapter=GreinerNon-LTE model atmosphere analysis of the supersoft X-ray source RX J0122.9-7521 |journalseries=Lect.Lecture Notes Phys.in Physics |editor=Greiner |volume=472 |publisherpages=Springer131–138 |___locationdoi=Berlin 10.1007/BFb0102256|pagesisbn=131978-3-540-61390-9 }}</ref>
==Nova==
There are three SSXSs with bolometric luminosity of ~10<sup>38</sup> erg/s that are novae: GQ Mus (BB, MW), V1974 Cyg (WD, MW), and Nova LMC 1995 (WD).<ref name=Greiner/> Apparently, as of 1999 the orbital period of Nova LMC 1995 if a binary was not known.
U Sco, a recurrent nova as of 1999 unobserved by [[ROSAT]], is a WD (74-7674–76 eV), L<sub>bol</sub> ~ (8-608–60) x 10<sup>36</sup> erg/s, with an orbital period of 1.2306 d.<ref name=Greiner/>
==Planetary nebula==
In the SMC, 1E 0056.8-7154 is a WD with bolometric luminosity of 2 x 10<sup>37</sup> that has a planetary nebula associated with it.<ref name=Greiner/>
==Super softSupersoft active galactic nuclei==
Supersoft active galactic nuclei reach luminosities up to 10<sup>45</sup> erg/s.<ref name=Greiner/>
==Large amplitude outbursts==
Large amplitude outbursts of super softsupersoft X-ray emission have been interpreted as [[tidal disruption event]]s.<ref name=Komossa>{{ cite journal |title=Discovery of a giant and luminous X-ray outburst from the optically inactive galaxy pair RX J1242.6-1119 |author=Komossa S|author2=Greiner J |journal=Astron. Astrophys. |volume=349 |date=1999 |pages=L45 |arxiv = astro-ph/9908216 |bibcode = 1999A&A...349L..45K }}</ref>
== See also ==
{{white dwarf}}
[[Category:Astronomical X-ray sources|*super softsupersoft X-ray source]]
[[Category:X-ray astronomy]]
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