LMC X-3
LMC X-3 is a high-mass X-ray binary system located in the Large Magellanic Cloud, a satellite galaxy of the Milky Way approximately 165,000 light-years away. 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.Discovery
Black Hole
LMC X-3 was first identified in 1971 by the Uhuru satellites lead by Leong et al. 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.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. In 1983, this was confirmed by Cowley et al. by using spectral and spectroscopic observation of LMC X-3.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.Orbital Parameters
The system has an orbital period of approximately 1.70481 days, with a separation of about 7 million miles. The orbit is inclined at 68°, 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.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 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.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.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.