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LMC X-2

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LMC X-2

Observation data
Epoch J2000      Equinox J2000
Constellation Mensa[1]
Right ascension 05h 20m 28.2s[2]
Declination −71° 57′ 33″[2]
Apparent magnitude (V) ~18.8[3]
Characteristics
Evolutionary stage Neutron star (A)
Evolved B-type star (B)
Spectral type Neutron star + ?
Astrometry
Radial velocity (Rv)262.2 ± 3.4[4] km/s
Distance~162,983 ly
(~49,970[5] pc)
Orbit
Primaryneutron star
Companionmain-sequence star
Period (P)0.32 ± 0.02 days[6]
 (8.15 hours)
Inclination (i)≲70[7]°
Semi-amplitude (K2)
(secondary)		315 ± 28[8] km/s
Details
A
Mass1.4 ± 0.6[9] M☉
Radius≲16 km[2] R☉
Temperature2,300,000 ± 900,000[2] K
B
Mass≤1[10] M☉
Radius6-10 km[11] R☉
Other designations
LMC X-2, SWIFT J0520.9-7156[12]
Database references
SIMBADdata

LMC X-2 is a low-mass X-ray binary (LMXB) located in the Large Magellanic Cloud (LMC), a satellite galaxy of Milky Way. It is one of the five brightest X-ray sources in the LMC and is the most luminous LMXB with a luminosity ranging from 0.3×1037 and 6×1038 ergs/s.[13] This binary system consist of a neutron star accreating matter from an orbiting low-mass blue star.[14] Its high luminosity is caused because of its X-ray emission is close to the Eddington limit or Eddington luminosity for a neutron star and because the Large Magellanic Cloud has lower metal abundances, allowing for higher Eddington luminosities and a higher accretion rate causing its high luminosity.[15][16][17]

LMC X-2 is classified as a Z-source, a subtype of neutron star LMXB characterized by high accretion rates and a distinctive Z-shaped track in X-ray color-color and hardness-intensity diagrams. These tracks reflect transitions between three spectral states: the horizontal branch, normal branch, and flaring branch. It is the first Z-source identified outside the Milky Way, making it the eighth known Z-source overall.[18][19]

Discovery

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Main system

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The main system of LMC X-2 was discovered around January 1971 by the Uhuru satellite during early satellite flights that identified the system as a point in the LMC along with 2 other points, which are now known as the supersoft X-ray binaries (SSXB) CAL 83 and CAL 87.[20][21]

Characteristics

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System Characteristics

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LMC X-2 consists of a neutron star and a companion evolved B-type star. The companion star orbits the neutron star in 8.15 hours from an inclination of around 70°,[7][6] its semi-major axis along with the eccentricity is unknown from the lack of orbital information and observations.

Physical Characteristics

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Neutron star

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This neutron star is the main star of the system LMC X-2, this has a diameter around 16 kilometers and a solar mass of 1.4 ± 0.6[2] by using the X-ray data of LMC X-2 and fit the data in a theoretical blackbody model which relates to the mass and radius along with its temperature.[22]

Companion star

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The companion star of LMC X-2 is quite unknown because of it being outshined by the neutron star it is orbiting, this only has information of the mass and radius of around 1 solar masses and 6 to 10 solar radii.[11][10]

Observation

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LMC X-2 is has been extensively studied by multiple X-ray observatories:

  • Four observations from Rossi X-ray Timing Explorer (RXTE) revealed the complete Z-diagram, confirming its Z-source classification through spectral state transitions and quasi-periodic oscillations.[23]
  • Archival data from XMM-Newton provided high-precision spectral analysis, with models incorporating blackbody emission (neutron star surface, ~1-2 keV) and Comptonization (corona). Luminosities indicate near-Eddington accretion.[24]
  • Approximately 140 ks of observations from Astrosat captured broad-band X-ray spectral evolution, showing changes in disk temperature, electron temperature, and optical depth along the Z-track.[25]
  • Other Observations from MAXI and Swift Space Telescope indicates persistent emission with occasional flares. No definitive orbital period has been identified despite photometric searches.[17]

See also

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References

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  1. ^ Roman, Nancy G. (1987). "Identification of a constellation from a position". Publications of the Astronomical Society of the Pacific. 99 (617): 695. Bibcode:1987PASP...99..695R. doi:10.1086/132034. Constellation record for this object at VizieR.
  2. ^ a b c d e Lavagetto, G.; Iaria, R.; d'Aì, A.; Di Salvo, T.; Robba, N. R. (2008). "Spectral analysis of LMC X–2 with XMM/Newton: unveiling the emission process in the extragalactic Z-source" (PDF). Astronomy & Astrophysics. 478: 181–186. Bibcode:2008A&A...478..181L. doi:10.1051/0004-6361:20078027. Retrieved 2025-10-18.
  3. ^ McGowan, K. E.; Charles, P. A.; O'Donoghue, D.; Smale, A. P. (November 2003). "Correlated optical and X-ray variability in LMC X-2". Monthly Notices of the Royal Astronomical Society. 345 (3): 1039–1048. arXiv:astro-ph/0307373. Bibcode:2003MNRAS.345.1039M. doi:10.1046/j.1365-8711.2003.07029.x.
  4. ^ "Kinematical Structure of the Magellanic System - R.P. van der Marel et al". ned.ipac.caltech.edu. Retrieved 2025-10-11.
  5. ^ Pietrzyński, G.; Graczyk, D.; Gieren, W.; Thompson, I. B.; Pilecki, B.; Udalski, A.; Soszyński, I.; Kozłowski, S.; Konorski, P.; Suchomska, K.; Bono, G.; Moroni, P. G. Prada; Villanova, S.; Nardetto, N.; Bresolin, F.; Kudritzki, R. P.; Storm, J.; Gallenne, A.; Smolec, R.; Minniti, D.; Kubiak, M.; Szymański, M. K.; Poleski, R.; Wyrzykowski, Ł.; Ulaczyk, K.; Pietrukowicz, P.; Górski, M.; Karczmarek, P. (2013). "An eclipsing-binary distance to the Large Magellanic Cloud accurate to two per cent". Nature. 495 (7439): 76–79. arXiv:1303.2063. Bibcode:2013Natur.495...76P. doi:10.1038/nature11878. Retrieved 2025-10-18.
  6. ^ a b Cornelisse, R.; Steeghs, D.; Casares, J.; Charles, P. A.; Shih, I. C.; Hynes, R. I.; O'Brien, K. (October 2007). "A signature of the donor star in the extra-galactic X-ray binary LMC X−2". Monthly Notices of the Royal Astronomical Society. 381 (1): 194–200. arXiv:0707.2018. Bibcode:2007MNRAS.381..194C. doi:10.1111/j.1365-2966.2007.12233.x. ISSN 0035-8711.
  7. ^ a b "Is the orbital period 8.15 h?". academic.oup.com. Retrieved 2025-10-18.
  8. ^ Cornelisse, R.; Steeghs, D.; Casares, J.; Charles, P. A.; Shih, I. C.; Hynes, R. I.; O'Brien, K. (2007). "A signature of the donor star in the extra-galactic X-ray binary LMC X−2". Monthly Notices of the Royal Astronomical Society. 381 (1): 194–200. arXiv:0707.2018. Bibcode:2007MNRAS.381..194C. doi:10.1111/j.1365-2966.2007.12233.x.
  9. ^ Smale, Alan P.; Kuulkers, Erik (2000). "LMC X-2: The First Extragalactic Z Source?". The Astrophysical Journal. 528 (2): 702–710. arXiv:astro-ph/9907303. Bibcode:2000ApJ...528..702S. doi:10.1086/308193.
  10. ^ a b Crampton, D.; Hutchings, J. B.; Cowley, A. P.; Schmidtke, P. C.; Thompson, I. B. (June 1990). "The low-mass X-ray binary LMC X-2". pp. 496–500.
  11. ^ a b Smale, Alan P.; Kuulkers, Erik (2000). "LMC X-2: The First Extragalactic Z Source?". The Astrophysical Journal. 528 (2): 702–710. arXiv:astro-ph/9907303. Bibcode:2000ApJ...528..702S. doi:10.1086/308193.
  12. ^ "Simbad - Object view". simbad.cds.unistra.fr. Retrieved 2025-10-08.
  13. ^ Bonnet-Bidaud, J. M.; Motch, C.; Beuermann, K.; Pakull, M.; Parmar, A. N.; Van Der Klis, M. (1989). "LMC X-2 : An extragalactic bulge-type source". Astronomy and Astrophysics. 213: 97. Bibcode:1989A&A...213...97B.
  14. ^ Ishioka, Ryoko; Chu 朱有, You-Hua 花.; Points, Sean D.; Li 李傳, Chuan-Jui 睿.; Chen, Chen-Hung (2020). "A Binary Star in the Superbubble N160 in the Large Magellanic Cloud". The Astrophysical Journal. 900 (2): 195. Bibcode:2020ApJ...900..195I. doi:10.3847/1538-4357/aba8f2.
  15. ^ Pakull, M. (1979). "Probable optical identification of LMC X-2". The Messenger. 16: 38. Bibcode:1979Msngr..16...38P.
  16. ^ [1]
  17. ^ a b Greene, J.; Wachter, S.; Smale, A. P. (May 1999). "LMC X-2: The Search for the Orbital Period". American Astronomical Society Meeting Abstracts #194. 194: 52.16. Bibcode:1999AAS...194.5216G.
  18. ^ Smale, Alan P.; Kuulkers, Erik (2000-01-10). "LMC X-2: The First Extragalactic Z Source?". The Astrophysical Journal. 528 (2): 702–710. arXiv:astro-ph/9907303. Bibcode:2000ApJ...528..702S. doi:10.1086/308193. ISSN 0004-637X.
  19. ^ "LMC X-2". maxi.riken.jp. Retrieved 2025-10-11.
  20. ^ https://academic.oup.com/mnras/article/316/4/729/1084536
  21. ^ Schmidtke, P. C.; Cowley, A. P.; Frattare, L. M.; McGrath, T. K.; Hutchings, J. B.; Crampton, D. (1994). "LMC Stellar X-Ray Sources Observed with ROSAT: I. X-Ray Data and Search for Optical Counterparts". Publications of the Astronomical Society of the Pacific. 106: 843. Bibcode:1994PASP..106..843S. doi:10.1086/133452.
  22. ^ Lavagetto, G.; Iaria, R.; d'Ai, A.; Di Salvo, T.; Robba, N. R. (2008). "Spectral Analysis of LMC-X2 with XMM/Newton". Chinese Journal of Astronomy and Astrophysics Supplement. 8: 325. Bibcode:2008ChJAS...8..325L.
  23. ^ White, Nicholas E.; Smale, A. P.; Homan, J.; Kuulkers, E. (2003-01-30). The Complete Z-diagram of LMC X-2 (Report).
  24. ^ Lavagetto, G.; Iaria, R.; D'Aì, A.; Salvo, T. Di; Robba, N. R. (2008-01-01). "Spectral analysis of LMC X–2 with XMM/Newton: unveiling the emission process in the extragalactic Z-source". Astronomy & Astrophysics. 478 (1): 181–186. arXiv:0710.4934. Bibcode:2008A&A...478..181L. doi:10.1051/0004-6361:20078027. ISSN 0004-6361.
  25. ^ Agrawal, V K; Nandi, Anuj (2020-09-21). "AstroSat view of LMC X-2: evolution of broad-band X-ray spectral properties along a complete Z-track". Monthly Notices of the Royal Astronomical Society. 497 (3): 3726–3733. arXiv:2007.05938. doi:10.1093/mnras/staa2063. ISSN 0035-8711.