Content deleted Content added
m →top: Effective link |
m Open access bot: doi added to citation with #oabot. |
||
Line 43:
===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. Soc. Pac. |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/>
Line 51:
==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. |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. |volume=488 |issue=2 |pages=827 |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. |date=1995 |volume=448 |pages=305 |bibcode=1995ApJ...448..305S}}</ref>
Line 57:
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.<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 |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.{{citation needed|date=August 2017}}<!-- Someone said Fleming, TA (1995), Astronomy 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/>
Line 79:
| bibcode = 1995MNRAS.277..971B
| doi = 10.1093/mnras/277.3.971
| doi-access= free
}}</ref>
| |||