Supersoft X-ray source#VY Scl cataclysmic variables

Luminous supersoft X-ray sources have a characteristic blackbody temperature of a few tens of eV (~20–100 eV){{ cite journal |author=Kahabka P |title=Supersoft X-ray sources |journal= Advances in Space Research|date=Dec 2006 |volume=38 |issue=12 |pages=2836–9 |doi=10.1016/j.asr.2005.10.058 |bibcode=2006AdSpR..38.2836K}} and a bolometric luminosity of ~10³⁸ erg/s (below ~ 3 x 10³⁸ erg/s).{{ 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= The 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}}

Apparently, luminous SSXSs can have equivalent blackbody temperatures as low as ~15 eV and luminosities ranging from 10³⁶ to 10³⁸ erg/s.{{ cite journal |author=Kahabka P|author2=van den Heuvel EPJ |title=Luminous Supersoft X-Ray Sources |journal= Annual Review 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 }} 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 10³.

SSXSs have now been discovered in our galaxy and in globular cluster M3. MR Velorum (RX J0925.7-4758) is one of the rare MW supersoft X-ray binaries.{{ cite journal |doi=10.1016/S1384-1076(00)00018-X |author=Greiner J |title=Catalog of supersoft X-ray sources |journal= New 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 }} "The source is heavily reddened by interstellar material, making it difficult to observe in the blue and ultraviolet."{{ 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= 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 }} 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.

The CBSS model invokes steady nuclear burning on the surface of an accreting white dwarf (WD) as the generator of the prodigious supersoft X-ray flux. As of 1999, eight SSXSs have orbital periods between ~4 hr and 1.35 d: RX J0019.8+2156 (MW), 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).

{{main|Symbiotic variable star}}

A symbiotic binary star is a variable binary star system in which a red giant has expanded its outer envelope and is shedding mass quickly, and another hot star (often a white dwarf) is ionizing the gas.{{ cite web |title=David Darling site symbiotic star description |url=http://www.daviddarling.info/encyclopedia/S/symbiotic_star.html }} Three symbiotic binaries as of 1999 are SSXSs: AG Dra (BB, MW), RR Tel (WD, MW), and RX J0048.4-7332 (WD, SMC).

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.{{ cite journal |author=Fleming TA|author2=Werner K |author3=Barstow MA |journal= 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 }}{{ cite journal |title=Spectral analysis of the hottest known helium-rich white dwarf: KPD 0005+5106 |author=Werner |journal= Astronomy and Astrophysics|date=1994 |volume=284 |pages=907 |bibcode = 1994A&A...284..907W }}

{{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."{{ cite journal |author=Kato T|author2=Ishioka R|author3=Uemura M |title=Photometric Study of KR Aurigae during the High State in 2001 |journal= Publications of 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}} Both fusion- and accretion-powered cataclysmic variables have been observed to be X-ray sources.{{ cite web |title=Introduction to Cataclysmic Variables (CVs) |url=http://heasarc.gsfc.nasa.gov/docs/objects/cvs/cvstext.html }} 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 Sagittae: bolometric luminosity of (1–10) x 10³⁷, a binary including a blackbody (BB) accretor at T < 80 eV, and an orbital period of 0.514195 d.

The accretion disk can become thermally stable in systems with high mass-transfer rates (Ṁ). Such systems are called nova-like (NL) stars, because they lack outbursts characteristic of dwarf novae.{{ cite journal |doi=10.1086/133689 |last1=Osaki |first1=Yoji |title=Dwarf-Nova Outbursts |journal= Publications of the Astronomical Society of the Pacific|date=1996 |volume=108 |pages=39 |bibcode=1996PASP..108...39O |doi-access=free }}

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.{{ cite book |author=Warner B |date=1995 |title=Cataclysmic Variable Stars |publisher=Cambridge University Press |location=Cambridge |bibcode=1995cvs..book.....W }}

V751 Cyg (BB, MW) is a VY Scl CV, has a bolometric luminosity of 6.5 x 10³⁶ erg/s, and emits soft X-rays at quiescence.{{ 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= Publications of the Astronomical Society of the Pacific|date=Jan 2001 |volume=113 |issue=779 |pages=72–81 |bibcode=2001PASP..113...72P |doi-access=free }} 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.

The high luminosity (6.5 x 10³⁶ 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). "A high luminosity in soft X-rays poses an additional problem of understanding why the spectrum is of only modest excitation." 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).

Pushing the edge of acceptable X-ray fits toward lower luminosity suggests that the luminosity should not exceed ~2 x 10³³ ergs/s, which gives only ~4 x 10³¹ ergs/s of reprocessed light in the WD about equal to the secondary's expected nuclear luminosity.

{{main|Polar (cataclysmic variable)}}

X-rays from magnetic cataclysmic variables are common because accretion provides a continuous supply of coronal gas. 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. The rotation has been blamed on asymmetric ejection of planetary nebulae and winds{{ cite journal |title=Origin of the rotation rates of single white dwarfs |author=Spruit HC |date=1998 |journal= Astronomy and Astrophysics|volume=333 |pages=603 |arxiv = astro-ph/9802141 |bibcode = 1998A&A...333..603S }} and the fields on in situ dynamos.{{ 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= The Astrophysical Journal|volume=488 |issue=2 |pages=827–830 |bibcode=1997ApJ...488..827S|doi-access=free }} Orbit and rotation periods are synchronized in strongly magnetized WDs. 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.{{ cite journal |doi=10.1086/175962 |author=Schmidt GD |author2=Smith PS |title=A Search for Magnetic Fields among DA White Dwarfs |journal= The Astrophysical Journal|date=1995 |volume=448 |pages=305 |bibcode=1995ApJ...448..305S|doi-access=free }}

Most highly magnetic WDs appear to be isolated objects, although G 23–46 (7.4 MG) and LB 1116 (670 MG) are in unresolved binary systems.

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.{{ 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= 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 }} 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.{{citation needed|date=August 2017}} RE J0317-853 is associated with a blue star 16 arcsec from LB 9802 (also a blue WD) but not physically associated. 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.

Until recently (1995) only PG 1658+441 possessed an effective temperature > 30,000 K. Its polar field strength is only 3 MG.

The ROSAT Wide Field Camera (WFC) source RE J0616-649 has an ~20 MG field.{{cite journal

| display-authors = 4| last1 = Barstow

| first1 = M. A.

| last2 = Jordan

| first2 = S.

| author3 = O'Donoghue, D.

| author4 = Burleigh, M. R.

| author5 = Napiwotzki, R.

| author6 = Harrop-Allin, M. K.

| name-list-style = amp

| date = December 1995

| title = RE J0317-853: the hottest known highly magnetic DA white dwarf

| journal = Monthly Notices of the Royal Astronomical Society

| volume = 277

| issue = 3

| pages = 971–985

| bibcode = 1995MNRAS.277..971B

| doi = 10.1093/mnras/277.3.971

| doi-access= free

}}

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^h24^m.{{ 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= The Astrophysical Journal|date=1987 |volume=320 |pages=308 |bibcode=1987ApJ...320..308L}}

The magnetic fields in CVs are confined to a narrow range of strengths, with a maximum of 7080 MG for RX J1938.4-4623.{{ cite journal |title=Two-pole accretion in the high-field polar RXJ 1938.6-4612 |author=Schwope AD|display-authors=etal |journal= Astronomy and Astrophysics|date=1995 |volume=293 |pages=764 |bibcode = 1995A&A...293..764S }}

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.

PG 1159 stars are a group of very hot, often pulsating WDs for which the prototype is PG 1159 dominated by carbon and oxygen in their atmospheres.{{ cite journal |author=Trimble V |title=White dwarfs in the 1990s |journal= Bulletin of the Astronomical Society of India|date=1999 |volume=27 |pages=549–66 |bibcode=1999BASI...27..549T }}

PG 1159 stars reach luminosities of ~10³⁸ erg/s but form a rather distinct class.{{ Cite book |author=Dreizler S|author2=Werner K|author3=Heber U |chapter=PG 1159 stars and their evolutionary link to DO white dwarfs |date=1995 |volume=443 |pages=160–170 |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}} RX J0122.9-7521 has been identified as a galactic PG 1159 star.{{ 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=Publ. Astron. Soc. Pac.|volume=107 |pages=927 |date=1995 |bibcode=1995PASP..107..927C|doi-access=free }}{{ Cite book |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-Ray Sources|chapter=Non-LTE model atmosphere analysis of the supersoft X-ray source RX J0122.9-7521 |series=Lecture Notes in Physics |editor=Greiner |volume=472 |pages=131–138 |doi=10.1007/BFb0102256|isbn=978-3-540-61390-9 }}

{{main|Nova}}

There are three SSXSs with bolometric luminosity of ~10³⁸ erg/s that are novae: GQ Mus (BB, MW), V1974 Cyg (WD, MW), and Nova LMC 1995 (WD). 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 (74–76 eV), L_bol ~ (8–60) x 10³⁶ erg/s, with an orbital period of 1.2306 d.

{{main|Planetary nebula}}

In the SMC, 1E 0056.8-7154 is a WD with bolometric luminosity of 2 x 10³⁷ that has a planetary nebula associated with it.

Supersoft active galactic nuclei reach luminosities up to 10⁴⁵ erg/s.

Large amplitude outbursts of supersoft X-ray emission have been interpreted as tidal disruption events.{{ 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 }}

• Carbon detonation
• Type Ia supernova
• X-ray astronomy

{{reflist|30em}}

{{white dwarf}}

*supersoft X-ray source