View source for RX J1856.5−3754

{{Short description|Star in the constellation Corona Australis}}
{{Starbox begin}}
{{Starbox image
|image = [[Image:RX J1856.5-3754.jpg|250px]]
|caption = X-ray image of RX J1856.5−3754
}}
{{Starbox observe
 |epoch=1996.7
|equinox=[[J2000.0]]<ref name="s1"/>
 |constell=[[Corona Australis]]
 |ra={{RA|18|56|35}}<ref name="s1">{{cite web|url=http://www.astro.sunysb.edu/fwalter/NS/ns.html|title=RX J185635-3754 - an Isolated Neutron Star|author=F. M. Walter|publisher=Department of Physics and Astronomy, [[State University of New York at Stony Brook]]|access-date=June 29, 2007}}</ref>
 |dec={{DEC|-37|54|36}}<ref name="s1"/>
 |appmag_v=~25.6<ref name="s1"/>
}}
{{Starbox detail
| mass=<math>1.5\pm0.2</math><ref name="Potekhin" />
| radius_km=<math>12.1^{+1.3}_{-1.6}</math><ref name="Potekhin" />
| age_myr                =   1  }}
{{Starbox catalog|names=RX&nbsp;J185635−3754, 1ES&nbsp;1853−37.9, 1RXS&nbsp;J185635.1−375433}}
{{Starbox reference |
  Simbad=1ES+1853-37.9 }}
{{Starbox end}}

'''RX J1856.5−3754''' (also called '''RX J185635−3754''', '''RX J185635−375''', and various other designations) is a [[neutron star]] in the [[constellation]] [[Corona Australis]]. At approximately 400 [[Light-year|light-years]] from Earth, it is the closest neutron star discovered to date.

==Discovery and location==
[[File:Zooming in on the very faint neutron star RX J1856.5-3754.ogg|thumb|left|Zooming in on the very faint neutron star RX J1856.5–3754]]
[[File:Isolated Neutron Star RX J185635-3754 - opo9732a.jpg|thumb|left|Hubble image of RX J1856.5−3754—the first direct observation of an isolated neutron star in visible light]]
RX J1856.5−3754 is thought to have formed in a [[supernova]] explosion of its [[companion star]] about one million years ago and is moving across the sky at 108&nbsp;km/s. It was discovered in 1992, and observations in 1996 confirmed that it is a neutron star, the closest to Earth discovered to date.<ref>{{cite book|last=Rees|first=Martin|title=Universe|url=https://books.google.com/books?id=IvDZqZz0Q-QC&pg=PA268|year=2012|publisher=Dorling Kindersley|isbn=978-1-4093-7650-7|page=528}}</ref>

It was originally thought to be about 150–200 [[light-year]]s away,<ref>[http://www.spaceref.com/news/viewpr.html?pid=2580 "The Mystery of the Lonely Neutron Star"]. [[European Southern Observatory]] press release, September 11, 2000. Accessed online at spaceref.com May 20, 2007.</ref> but further observations using the [[Chandra X-ray Observatory]] in 2002 indicate that its distance is greater—about 400 light-years.<ref name="Drake2002"/><ref name="Ho2007"/>

RX J1856 is one of [[The Magnificent Seven (neutron stars)|the Magnificent Seven]], a group of young neutron stars at distances between {{convert|130|and|500|pc|abbr=off|lk=in}} of Earth.

== Quark star hypothesis ==
By combining Chandra X-ray Observatory and [[Hubble Space Telescope]] data, astronomers previously estimated that RX J1856 radiates like a solid body with a temperature of 700,000 °C and has a diameter of about 4–8&nbsp;km. This estimated size was too small to reconcile with the standard models of neutron stars, and it was therefore suggested that it might be a [[quark star]].<ref name="Drake2002">{{cite journal
 | author = Drake J. J.
 | display-authors = etal
 | title = Is RX J1856.5-3754 a Quark Star?
 | journal = Astrophys. J.
 | date = 2002
 | volume = 572
 | issue = 2
 | pages = 996–1001
 | doi = 10.1086/340368
|arxiv = astro-ph/0204159 |bibcode = 2002ApJ...572..996D | s2cid = 18481546
 }}</ref>

However, later refined analysis<ref name="Ho2007"/><ref name="Truemper2004"/> of improved Chandra and Hubble observations revealed that the surface temperature of the star is lower, only 434,000&nbsp;°C, and, respectively, the radius is larger, about 14&nbsp;km (the observed radius  with account of the effects of [[general relativity]] appears about 17&nbsp;km).<ref name="Ho2007">{{cite journal
 | author = Ho W. C. G.
 | display-authors = etal
 | title = Magnetic hydrogen atmosphere models and the neutron star RX J1856.5–3754
 | journal = [[Monthly Notices of the Royal Astronomical Society]]
 | date = 2007
 | volume = 375
 | issue = 2
 | pages = 821–830
 | doi = 10.1111/j.1365-2966.2006.11376.x
 | doi-access = free
 | arxiv = astro-ph/0612145v1 |bibcode = 2007MNRAS.375..821H | s2cid = 2046995
 }}</ref> Thus, RX J1856.5–3754 is now excluded from the list of quark star candidates.<ref name="Truemper2004">{{cite journal
 | author = Truemper, J. E.
 | author2 = Burwitz, V.
 | author3 = Haberl, F.
 | author4 = Zavlin, V. E.
 | title = The puzzles of RX J1856.5-3754: neutron star or quark star?
 | journal = Nuclear Physics B: Proceedings Supplements
 | date = June 2004
 | volume = 132
 | pages = 560–565
 | doi = 10.1016/j.nuclphysbps.2004.04.094
 | bibcode = 2004NuPhS.132..560T
|arxiv = astro-ph/0312600 | s2cid = 425112
 }}</ref>
A subsequent more accurate parallax estimation has led to the correction of this result to <math>12.1^{+1.3}_{-1.6}</math> km for the true radius (and about 15 km for the observed radius).<ref name="Potekhin">{{Citation |last=Potekhin |first=Alexander Y. |year=2014 |title=Atmospheres and radiating surfaces of neutron stars |journal= [[Physics-Uspekhi|Phys. Usp.]] |volume=57 |pages=735–770 |url=https://ufn.ru/en/articles/2014/8/a/ |access-date=2024-12-04}}</ref>

==Vacuum birefringence==
[[File:The polarisation of light emitted by a neutron star.jpg|thumb|The polarization of the observed light in the extremely strong magnetic field suggests that the empty space around the neutron star is subject to the [[Euler–Heisenberg Lagrangian|vacuum birefringence.]]<ref>{{cite web|title=First Signs of Weird Quantum Property of Empty Space? – VLT observations of neutron star may confirm 80-year-old prediction about the vacuum|url=https://www.eso.org/public/news/eso1641/|website=www.eso.org|access-date=5 December 2016}}</ref>]]

In 2016 a team of astronomers from Italy, Poland, and the U.K. using the [[Very Large Telescope]] reported<ref>{{cite journal|last1=Mignani|first1=R. P.|last2=Testa|first2=V.|last3=González Caniulef|first3=D.|last4=Taverna|first4=R.|last5=Turolla|first5=R.|last6=Zane|first6=S.|last7=Wu|first7=K.|date=2016-11-02|title=Evidence for vacuum birefringence from the first optical-polarimetry measurement of the isolated neutron star RX J1856.5−3754|journal=Monthly Notices of the Royal Astronomical Society|publisher=Oxford University Press (OUP)|volume=465|issue=1|pages=492–500|arxiv=1610.08323|doi=10.1093/mnras/stw2798|doi-access=free |issn=0035-8711}}</ref><ref>{{Cite web|title=Astronomers Report First Observational Evidence for Vacuum Birefringence {{!}} Astronomy {{!}} Sci-News.com|url=http://www.sci-news.com/astronomy/vacuum-birefringence-04410.html|access-date=2021-10-10|website=Breaking Science News {{!}} Sci-News.com|language=en-US}}</ref> observational indications of [[vacuum birefringence]] from RX J1856.5−3754. A [[degree of polarization]] of about 16% was measured from the visible spectrum being large enough to support evidence but not discovery due to the low accuracy of star model and the uncertain direction of the neutron magnetization axis.<ref>{{cite journal|last1=Fan|first1=Xing|last2=Kamioka|first2=Shusei|last3=Inada|first3=Toshiaki|last4=Yamazaki|first4=Takayuki|last5=Namba|first5=Toshio|last6=Asai|first6=Shoji|last7=Omachi|first7=Junko|last8=Yoshioka|first8=Kosuke|last9=Kuwata-Gonokami|first9=Makoto|last10=Matsuo|first10=Akira|last11=Kawaguchi|first11=Koushi|display-authors=5|year=2017|title=The OVAL experiment: a new experiment to measure vacuum magnetic birefringence using high repetition pulsed magnets|journal=The European Physical Journal D|publisher=Springer Nature|volume=71|issue=11|page=308|arxiv=1705.00495|bibcode=2017EPJD...71..308F|doi=10.1140/epjd/e2017-80290-7|issn=1434-6060|first13=Hiroyuki|last13=Nojiri|first12=Koichi|last12=Kindo|s2cid=254140737 }}</ref> 

Its inferred magnetic effect of 10<sup>13</sup>&nbsp;[[Gauss (unit)|G]] should produce a greater effect at X-ray wavelength which could be measured by future planned [[Polarimeter|polarimeters]] such as [[NASA]]'s [[IXPE|Imaging X-ray Polarimeter Explorer (IXPE)]], NASA's Polarimetry of Relativistic X-ray Sources (PRAXYS) or [[European Space Agency|ESA]]'s X-ray Imaging Polarimetry Explorer (XIPE).<ref>{{Cite news|date=2016-12-07|title=Does This Neutron Star Confirm an 80-Year-Old Quantum Prediction?|url=https://skyandtelescope.org/astronomy-news/neutron-star-confirm-80-year-old-quantum-prediction/|access-date=2021-10-10|website=Sky & Telescope|language=en-US}}</ref>

==See also== 
*[[3C 58]], a possible quark star

==References==
* [http://jumk.de/astronomie/special-stars/rx-j185635-375.shtml RX-J185635-375] at jumk.de
* [http://chandra.harvard.edu/photo/2002/0211/ RX J1856.5-3754 and 3C58: Cosmic X-rays May Reveal New Form of Matter] [[Chandra X-ray Observatory]]. July 16, 2009.
* Walter, Frederick M.; Lattimer, James M., ''The Astrophysical Journal'', 2002
{{Reflist}}

==External links==
{{commons}}
* [https://arxiv.org/abs/astro-ph/0204159 Is RX J185635-375 a Quark Star?]
* {{APOD |date=14 April 2002 |title=RX J185635-375: Candidate Quark Star}}
* [http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2004ApJ...603..265T Bare Quark Stars or Naked Neutron Stars? The Case of RX J1856.5-3754]
* [http://www.astro.sunysb.edu/fwalter/NS/ns.html RX J185635-3754 - an Isolated Neutron Star]
* [http://hubblesite.org/newscenter/archive/releases/star/neutron-star/1997/32/image/a/ News Release STScI-1997-32: Hubble Sees a Neutron Star Alone in Space]

{{Corona Australis}}

{{DEFAULTSORT:RX J1856.5-3754}}
[[Category:Corona Australis]]
[[Category:Radio-quiet neutron stars]]
[[Category:Neutron stars]]
[[Category:ROSAT objects]]