View source for Scorpius X-1

{{Short description|Astronomical X-ray source}}
{{Starbox short |
 name=V818 Sco or Scorpius X-1 |
 epoch=[[J2000.0]] |
 constell=[[Scorpius]] |
 ra=16<sup>h</sup> 19<sup>m</sup> 55.07<sup>s</sup><ref name="simbad">{{cite web
  | author=Staff | date=March 3, 2003
  | url=https://simbad.u-strasbg.fr/simbad/sim-id?Ident=sco+x-1
  | title=V* V1357 Cyg—High Mass X-ray Binary
  | publisher=Centre de Données astronomiques de Strasbourg
  | accessdate=2009-12-21}}</ref> |
 dec=−15° 38' 24.8"<ref name="simbad" /> |
 spectral= |
 appmag_v=12.2<ref name="simbad" /> |
 dist_ly=9,000 |
 dist_pc=2,800<ref name="par">{{cite journal | author = Bradshaw, C.F.| author2 = Fomalont, E.B.| author3 = Geldzahler, B.J. | title = High-Resolution Parallax measurements of Scorpius X-1 | journal = The Astrophysical Journal | volume = 512 | issue = 2 | pages = L121–L124 | bibcode = 1999ApJ...512L.121B | doi = 10.1086/311889 | date = 1999 | doi-access = free }}</ref> |
 names= V818 Sco, [[List of astronomical catalogues#H|H]] 1620-15, [[2RE]] J161955-153824, [[1XRS]] 16170-155, [[Ariel V SSI catalogue|2A]] 1616-155, [[INTEGRAL1]] 21, [[ROSAT catalog|RE]] J1619-153, [[RXTE catalog|XSS]] J16204-1536, [[Ariel V 3A catalog|3A]] 1617-155, [[INTREF]] 685, [[ROSAT catalog|RE]] J161956-153814, X Sco X-1, [[2EUVE]] J1619-15.6, [[KOHX]] 20, [[SBC7]] 569, [[EUVE]] J1619-15.6, 1M 1617-155, [[Second UHURU catalog|2U]] 1617-15, [[AAVSO]] 1614-15, 1H 1617-155, [[2MASS]] J16195506-1538250, [[Third UHURU catalog|3U]] 1617-15, [[List of astronomical catalogues#H|H]] 1617-155, [[2RE]] J1619-153, [[Fourth UHURU catalog|4U]] 1617-15
| Simbad = V818+Sco
}}

'''Scorpius X-1''' is an [[X-ray]] source located roughly 9000 [[light year]]s away in the constellation [[Scorpius (constellation)|Scorpius]]. Scorpius X-1 was the first extrasolar X-ray source discovered, and, aside from the [[Sun]], it is the strongest apparent non-transient source of X-rays in the [[sky]].<ref>{{Cite journal | doi=10.1103/PhysRevLett.9.439 | last1=Giacconi | first1=R. | last2=Gursky | first2=H. | last3=Paolini |first3=F.R. | last4=Rossi | first4=B.B. | title=Evidence for X-rays from sources outside the solar system | journal=Phys. Rev. Lett. | volume=9 | issue=11 | pages=439–443 | date=1962 | bibcode=1962PhRvL...9..439G| doi-access=free }}</ref> The X-ray [[flux]] varies day-to-day, and is associated with an [[visible light|optically]] visible [[star]], '''V818 Scorpii''', that has an [[apparent magnitude]] which fluctuates between 12–13.<ref name="shklovskii">{{cite book | author = Shklovskii, Iosif S. | title = Stars: Their Birth, Life, and Death | date = 1978 | publisher = W.H. Freeman | isbn = 978-0-7167-0024-1 | url = https://archive.org/details/starstheirbirthl00shkl| url-access = registration }}</ref>

==Discovery and early study==
The possible existence of cosmic soft X-rays was first proposed by [[Bruno Rossi]], MIT professor and board chairman of [[American Science and Engineering]] in [[Cambridge, Massachusetts]] to Martin Annis, president of AS&E. Following his urging, the company obtained a contract from the United States Air Force to explore the lunar surface prior to the launch of astronauts to the Moon, and incidentally to perhaps see galactic sources of X-rays.

Subsequently, Scorpius X-1 was discovered in 1962 by a team, under [[Riccardo Giacconi]], who launched an [[Aerobee]] 150 sounding rocket carrying a highly sensitive soft X-ray detector designed by [[Frank Paolini]].  The rocket trajectory was slightly off course but still detected a significant emission of soft X-rays that were not coming from the Moon. Thus fortuitously, and as first pointed out by Frank Paolini, Scorpius X-1 became the first X-ray source discovered outside the [[Solar System]]. The [[angular resolution]] of the detector did not initially allow the position of Scorpius X-1 to be accurately determined. This led to suggestions that the source might be located near the [[Galactic Center]], but it was eventually realized that it lies in the constellation Scorpius.<ref name="shklovskii" /> As the first discovered X-ray source in Scorpius, it received the designation Scorpius X-1.

The [[Aerobee]] 150 rocket launched on June 12, 1962, detected the first X-rays from another celestial source (Scorpius X-1) at J1950 [[Right ascension|RA]] {{RA|16|15}} [[Declination|Dec]] {{Dec|-15.2}}.<ref name=Giacconi>{{cite journal |author=Giacconi R |title=Nobel Lecture: The dawn of X-ray astronomy |journal=Rev. Mod. Phys. |date=August 2003 |volume=75 |issue=3 |pages=995–1010 |doi=10.1103/RevModPhys.75.995 |bibcode=2003RvMP...75..995G|doi-access=free }}</ref> Sco X-1 is a [[LMXB]] in which the visual counterpart is V818 Scorpii.

Although the above reference indicates the rocket launch was on June 12, 1962, other sources indicate the actual launch was at 06:59:00 UTC on June 19, 1962.<ref name=Drake>{{cite web |author=Drake SA |title=A Brief History of High-Energy Astronomy: 1960–1964 |date=September 2006 |url=http://heasarc.gsfc.nasa.gov/docs/heasarc/headates/1960.html }}</ref><ref name=USAF6>{{cite web |title=Chronology—Quarter 2 1962 |url=http://www.astronautix.com/chrono/19622.htm |url-status=dead |archiveurl=https://web.archive.org/web/20100118132318/http://astronautix.com/chrono/19622.htm |archivedate=2010-01-18 }}</ref>

Historical footnote:
"The instrumentation had been designed for an attempt to observe X-rays from the moon and was not equipped with collimation to restrict the field of view narrowly. As a result, the signal was very broad, and accurate definition of the size and position of the source was not possible. A similar experiment was repeated in October 1962 when the Galactic Center was below the horizon and the strong source was not present. A third attempt, in June 1963, verified the results of the June 1962 flight."<ref name=Bowyer/> The [[Galactic Center]] is < 20° [[Right ascension|RA]] and < 20° [[Declination|Dec]] from Sco X-1, the two X-ray sources are separated by ~20° of arc and may not have been resolvable in the June 1962 flight.<ref name=Bowyer/>

Scorpius XR-1 has been observed at J1950 [[Right ascension|RA]] {{RA|16|15}} [[Declination|Dec]] {{Dec|-15.2}}.<ref name=Bowyer>{{cite journal |author=Bowyer S|author2= Byram ET|author3=Chubb TA|author4=Friedman H |title=Observational results of X-ray astronomy |journal=Astronomical Observations from Space Vehicles, Proceedings from Symposium No. 23 Held in Liège, Belgium, 17 to 20 August 1964. |volume= 23|date=1965 |editor=Steinberg JL |publisher=[[International Astronomical Union]] |pages=227–39 |bibcode=1965IAUS...23..227B }}</ref>

In 1967 (before the discovery of [[pulsar]]s), [[Iosif Shklovsky]] examined X-ray and optical observations of Scorpius X-1 and correctly concluded that the radiation comes from a [[neutron star]] accreting matter from a companion.<ref>{{Cite journal | last=Shklovsky | first=I.S. | title=On the Nature of the Source of X-Ray Emission of SCO XR-1 | journal=Astrophys. J. | volume=148 | issue=1 | pages=L1–L4 | date=April 1967 | doi=10.1086/180001 | bibcode=1967ApJ...148L...1S}}</ref>

==Characteristics==
[[File:V818ScoLightCurve.png|thumb|left|A broadband optical [[light curve]] for V818 Scorpii, adapted from Hynes ''et al.'' (2016)<ref name="Hynes"/>]]
Its X-ray output is 2.3×10<sup>31</sup> [[Watt|W]], about 60,000 times the total luminosity of the Sun.<ref name="par" /> Scorpius X-1 shows regular variations of up to 1 [[apparent magnitude|magnitude]] in its intensity, with a period of around 18.9 hours. The source varies irregularly in optical [[wavelength]]s as well, but these changes are not correlated with the X-ray variations.<ref name="shklovskii" /> Scorpius X-1 itself is a [[neutron star]] whose intense gravity draws material off its companion into an [[accretion disk]], where it ultimately falls onto the surface, releasing a tremendous amount of energy. As this stellar material accelerates in Scorpius X-1's gravitational field, X-rays are emitted. The measured luminosity for Scorpius X-1 is consistent with a neutron star which is accreting matter at its [[Eddington limit]].<ref name ="par" />

This system is classified as a [[low-mass X-ray binary]]; the neutron star is roughly 1.4 [[solar masses]], while the donor star is only 0.42 solar masses.<ref>{{cite journal | author = Steeghs, D.| author2 = Casares, J. | title = The Mass Donor of Scorpius X-1 Revealed | journal = The Astrophysical Journal | volume = 568 | issue = 1 | pages = 273–278 | date = 2002 | doi = 10.1086/339224 | bibcode = 2002ApJ...568..273S |arxiv = astro-ph/0107343 | s2cid = 14136652 }}</ref> The origin of the system is a matter of debate. There is evidence that the two stars were not born together; studies based on the reconstruction of the orbit of Sco X-1 suggest that the binary may have been formed by a close encounter inside a [[globular cluster]]. However, it is not clear how to reconcile this formation scenario with the circularisation of the binary's orbit.<ref>{{cite journal | author = Mirabel, I. F.| author2 = Rodrogues, I. | title = The origin of Scorpius X-1 | journal = Astronomy and Astrophysics | volume = 398 | issue = 3 | pages = L25–L28 | date = 2003 | doi = 10.1051/0004-6361:20021767 | bibcode =2003A&A...398L..25M |arxiv = astro-ph/0301580 | s2cid = 2585509 }}</ref>

==See also==
* [[List of X-ray pulsars]]

== References ==
{{reflist|2|refs=

<ref name="Hynes">{{cite journal |last1=Hynes |first1=Robert I. |last2=Schaefer |first2=Bradley E. |last3=Baum |first3=Zachary A. |last4=Hsu |first4=Ching-Cheng |last5=Cherry |first5=Michael L. |last6=Scaringi |first6=Simone |title=Kepler K2 observations of Sco X-1: orbital modulations and correlations with Fermi GBM and MAXI |journal=Monthly Notices of the Royal Astronomical Society |date=July 2016 |volume=459 |issue=4 |pages=3596–3613 |doi=10.1093/mnras/stw854 |arxiv=1605.00546 |bibcode=2016MNRAS.459.3596H |doi-access=free }}</ref>

}}

{{Sky|16|19|55.07|-|15|38|24.8|9000}}

{{Stars of Scorpius}}

[[Category:Neutron stars]]
[[Category:X-ray binaries]]
[[Category:Variable stars]]
[[Category:Scorpius]]
[[Category:Astronomical X-ray sources]]
[[Category:Objects with variable star designations|Scorpii, V818]]