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LMC X-3
X-ray binaries in Large Magellanic Cloud From Wikipedia, the free encyclopedia
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LMC X-3 is a high-mass X-ray binary (HMXB) system located in the Large Magellanic Cloud (LMC), a satellite galaxy of the Milky Way approximately 165,000 light-years (50.5 kiloparsecs) away.[7] The system consists of a stellar-mass black hole accreting material from a B-type main-sequence companion star, producing intense X-ray emission via a hot accretion disk. LMC X-3 is one of the most studied extragalactic black hole binaries due to its brightness and variability.[8][9][10]
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Discovery
Black Hole
LMC X-3 was first identified in 1971 by the Uhuru satellites lead by Leong et al.[11] and discovered as a bright X-ray source in the Large Magellanic Cloud. This was identified as a black hole in 1983 by Anne Cowley et al. using dynamic observations along with the companion B-star.[12]
Star
The companion of the LMC X-3 black hole was located on 1975 by Rick Warren and Jeffrey Penfold when they saw an optical counterpart as a OB star in the X-ray error circle.[13] In 1983, this was confirmed by Cowley et al. by using spectral and spectroscopic observation of LMC X-3.[14]
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Characterstics
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LMC X-3 comprises a black hole and a B-type companion star, classified as B2.5Ve. The companion, with a surface temperature significantly hotter than the Sun’s, transfers mass to the black hole via Roche-lobe overflow, forming an accretion disk that emits X-rays up to 10,000 times the Sun’s total luminosity.[15]
Orbital Parameters
The system has an orbital period of approximately 1.70481 days, with a separation of about 7 million miles (11 million kilometers). The orbit is inclined at 68° (+2°/−3°), preventing eclipses. The companion’s radial velocity semi-amplitude is 256.7 ± 4.9 km/s, yielding a mass function of ~2.3 solar masses. Optical light curves show double-humped profiles due to the companion’s ellipsoidal distortion.[16][17]
Variability
LMC X-3 is notable for its persistent yet highly variable nature, often remaining in soft spectral states dominated by thermal disk emission, making it ideal for testing accretion disk models. It exhibits long-term intensity variations on 100–300 day timescales and enters anomalous low states (ALS) lasting 80+ days, during which X-ray and UV brightness drops significantly, with reduced variability. These ALS events, observed multiple times, are likely driven by changes in mass accretion rate from the companion, with X-ray lags of about 8 days during state transitions. The inner disk radius remains remarkably constant across observations, supporting reliable spin measurements via continuum fitting.[18][19][20][21]
Spin and polarization
The black hole's spin parameter is low, estimated at ~0.2 using X-ray continuum fitting. In 2023, IXPE detected X-ray polarization with a polarization degree of 3.2% ± 0.6% and a polarization angle of −42° ± 6° in the 2–8 keV band, setting an upper spin limit of a < 0.7 at 90% confidence. Polarization increases slightly with energy, consistent with other soft-state black hole binaries. Simultaneous NICER and NuSTAR observations confirmed the soft-state nature and spin estimates.[22]
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Formation and Evolution
Evolutionary models suggest LMC X-3 formed from a zero-age main-sequence binary, evolving through a supernova explosion of the primary star, with the current phase involving stable mass transfer. Its proximity to the transient/persistent divide among black hole X-ray binaries makes it a key system for understanding accretion physics and black hole formation in low-metallicity environments like the LMC.[23]
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Reference
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