LMC X-3
X-ray binaries in Large Magellanic Cloud
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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.[4] 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.[5][6][7]
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| Observation data Epoch J2000 Equinox J2000 | |
|---|---|
| Constellation | Dorado |
| Right ascension | 05h 38m 56s |
| Declination | −64° 05′ 03″ |
| Apparent magnitude (V) | 16.7 |
| Characteristics | |
| Evolutionary stage | Main-sequence (A) Black hole (B) |
| Spectral type | BH + B2.5Ve |
| Astrometry | |
| Distance | 165,000[1] ly (50,589 pc) |
| Orbit | |
| Primary | main-sequence star |
| Name | black hole |
| Period (P) | 1.70481 days |
| Semi-major axis (a) | 11000000 km[1] |
| Eccentricity (e) | 0.22 ± 0.4[2] |
| Inclination (i) | 69.8° ± 0.84[3]° |
| Semi-amplitude (K2) (secondary) | 256.7 ± 4.9 km/s |
| Details[4] | |
| A | |
| Mass | 3.63 ± 0.57 M☉ |
| Radius | 4.4 R☉ |
| B | |
| Mass | 6.98 ± 0.56 M☉ |
| Other designations | |
| LMC X-3, RX J0538.9-6405, 2MAXI J0539-640, 2XMM J053856.5-640503, Gaia DR3 4757068874690668160[5] | |
| Database references | |
| SIMBAD | data |
Discovery
Black Hole
LMC X-3 was first identified in 1971 by the Uhuru satellites lead by Leong et al.[8] 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.[9]
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.[10] In 1983, this was confirmed by Cowley et al. by using spectral and spectroscopic observation of LMC X-3.[8]
Characteristics
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.[11]
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.[12][13]
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.[14][15][16][17]
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.[18]
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.[19]