Editing Small Magellanic Cloud (section)

==X-ray sources==
The Small Magellanic Cloud contains a large and active population of [[X-ray binary|X-ray binaries]]. Recent star formation has led to a large population of massive stars and high-mass X-ray binaries (HMXBs), which are the relics of the short-lived upper end of the [[initial mass function]]. The young stellar population and most known X-ray binaries are concentrated in the SMC's Bar.
HMXB pulsars are rotating neutron stars in binary systems with Be-type ([[spectral type]] 09-B2, [[luminosity class]]es V–III) or [[supergiant]] stellar companions. Most HMXBs are of the Be type which account for 70% in the Milky Way and 98% in the SMC.<ref>Coe et al. 2005</ref> The Be-star equatorial disk provides a reservoir of matter that can be accreted onto the neutron star during [[periastron]] passage (most known systems have large orbital eccentricity) or during large-scale disk ejection episodes. This scenario leads to strings of X-ray outbursts with typical X-ray luminosities L<sub>x</sub>&nbsp;=&nbsp;10<sup>36</sup>–10<sup>37</sup>&nbsp;[[erg]]/s, spaced at the orbital period, plus infrequent giant outbursts of greater duration and luminosity.<ref>Negueruela 1998 for a review</ref>

Monitoring surveys of the SMC performed with NASA's [[Rossi X-ray Timing Explorer]] (RXTE)<ref>Laycock et al. 2005; Galache et al. 2008</ref> see X-ray pulsars in outburst at more than 10<sup>36</sup>&nbsp;erg/s and have counted 50 by the end of 2008. The ROSAT and ASCA missions detected many faint X-ray point sources,<ref>Haberl & Sasaki 2000</ref> but the typical positional uncertainties frequently made identification difficult. Recent studies using XMM-Newton<ref>Haberl et al. 2008; Haberl & Pietsch 2004</ref> and Chandra<ref>Antoniou et al. 2009; Edge et al. 2004, and Laycock et al. 2010</ref> have now cataloged several hundred X-ray sources in the direction of the SMC, of which perhaps half are considered likely HMXBs, and the remainder a mix of foreground stars, and background AGN.

No X-rays above background were observed from the Magellanic Clouds during the September 20, 1966, [[Nike-Tomahawk]] flight.<ref name=Chodil>{{cite journal |display-authors=4|author=Chodil G|author2=Mark H |author3=Rodrigues R |author4=Seward FD |author5=Swift CD |title=X-Ray Intensities and Spectra from Several Cosmic Sources |journal=Astrophys. J. |date=Oct 1967 |volume=150 |issue=10 |pages=57–65 |bibcode=1967ApJ...150...57C |doi=10.1086/149312|doi-access=free }}</ref> Balloon observation from Mildura, Australia, on October 24, 1967, of the SMC set an upper limit of X-ray detection.<ref name=Lewin>{{cite journal |author=Lewin WHG |author2=Clark GW |author3= Smith WB |title=Search for X-rays from the Large and Small Magellanic Clouds |date=1968 |journal=Nature |volume=220 |issue=5164 |pages=249–250 |bibcode=1968Natur.220..249L |doi=10.1038/220249b0 |s2cid=4187949 }}</ref> An X-ray astronomy instrument was carried aboard a [[PGM-17 Thor|Thor]] missile launched from [[Johnston Atoll]] on September 24, 1970, at 12:54 UTC for altitudes above 300&nbsp;km, to search for the Small Magellanic Cloud.<ref name=Price>{{cite journal |author=Price RE |display-authors=4 |author2=Groves DJ |author3=Rodrigues RM |author4=Seward FD |author5=Swift CD |author6=Toor A |title=X-Rays from the Magellanic Clouds |journal=Astrophys. J. |date=Aug 1971 |volume=168 |issue=8 |pages=L7–9 |bibcode=1971ApJ...168L...7P |doi=10.1086/180773 |doi-access=free }}</ref> The SMC was detected with an X-ray luminosity of 5{{e|38}}&nbsp;erg/s in the range 1.5–12&nbsp;keV, and 2.5{{e|39}}&nbsp;erg/s in the range 5–50&nbsp;keV for an apparently extended source.<ref name=Price/>

The fourth [[Uhuru (satellite)|Uhuru]] catalog lists an early X-ray source within the [[constellation]] [[Tucana]]: 4U 0115-73 (3U 0115-73, 2A 0116-737, SMC X-1).<ref name=Forman>{{cite journal |author=Forman W |display-authors= 4 |author2= Jones C |author3=Cominsky L |author4=Julien P |author5=Murray S |author6=Peters G |title=The fourth Uhuru catalog of X-ray sources |journal= Astrophys. J. Suppl. Ser. |date=1978 |volume=38 |page=357 |doi=10.1086/190561 |bibcode=1978ApJS...38..357F|doi-access=free }}</ref> Uhuru observed the SMC on January 1, 12, 13, 16, and 17, 1971, and detected one source located at 01149-7342, which was then designated SMC X-1.<ref name=Leong>{{cite journal |author=Leong C |display-authors=4 |author2=Kellogg E |author3=Gursky H |author4=Tananbaum H |author5= Giacconi R |title=X-Ray Emission from the Magellanic Clouds Observed by UHURU |journal=Astrophys. J. |date=Dec 1971 |volume=170 |issue=12 |pages=L67–71 |bibcode=1971ApJ...170L..67L |doi=10.1086/180842 }}</ref> Some X-ray counts were also received on January 14, 15, 18, and 19, 1971.<ref name=Tananbaum73>{{cite book |author=Tananbaum HD |title=X- and Gamma-Ray Astronomy |chapter=UHURU Results on Galactic X-ray Sources |volume=55 |editor=Bradt H |editor2= Giacconi R |publisher=International Astronomical Union |pages=9–28 |bibcode=1973IAUS...55....9T |location=Dordrecht, Holland |year=1973 |doi=10.1007/978-94-010-2585-0_2 |isbn=978-90-277-0337-8 }}</ref> The third [[Ariel 5]] catalog (3A) also contains this early X-ray source within Tucana: 3A 0116-736 (2A 0116-737, SMC X-1).<ref name=McHardy81>{{cite journal |author=McHardy IM |author2= Lawrence A |author3= Pye JP |author4=Pounds KA |title=The Ariel V /3 A/ catalogue of X-ray sources. II - Sources at high galactic latitude /absolute value of B greater than 10 deg/ |journal=Monthly Notices of the Royal Astronomical Society |date=Dec 1981 |volume=197 |issue= 4 |pages=893–919 |bibcode=1981MNRAS.197..893M |doi=10.1093/mnras/197.4.893|doi-access=free }}</ref> The SMC X-1, a HMXRB, is at J2000 [[right ascension]] (RA) {{RA|01|15|14}} [[declination]] (Dec) {{Dec|73|42|22}}.

Two additional sources detected and listed in 3A include SMC X-2 at 3A 0042-738 and SMC X-3 at 3A 0049-726.<ref name=McHardy81/>