X1822–371, associated with the optically visible

star A star is an astronomical object comprising a luminous spheroid of plasma (physics), plasma held together by its gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked ...

V691 Coronae Australis (abbreviated V691 CrA), is a neutron-star X-ray binary system at a distance of approximately 2-2.5 kiloparsecs. It is known to have a high inclination of i = 82.5°± 1.5°. This source displays relatively high brightness in the optical wavelengths when compared to the X-ray, making it a prototypical Accretion Disk Coronae (ADC) source, i.e. a source with a corona extending above and below its accretion disk. The only-partial eclipses in its light curve, even at such a high inclination, support this hypothesis. Estimates of the mass of its neutron star lies between 1.14–2.32

solar mass The solar mass () is a standard unit of mass in astronomy, equal to approximately . It is often used to indicate the masses of other stars, as well as stellar clusters, nebulae, galaxies and black holes. It is approximately equal to the mass ...

es. The optical spectrum of X1822–371 displays strong

Hα H-alpha (Hα) is a specific deep-red visible spectral line in the Balmer series with a wavelength of 656.28 nm in air and 656.46 nm in vacuum; it occurs when a hydrogen electron falls from its third to second lowest energy level. H-alpha ...

, Hβ, He I, He II and Bowen Blend features. These features have been extensively studied using the technique of

Doppler tomography An inverse problem in science is the process of calculating from a set of observations the causal factors that produced them: for example, calculating an image in X-ray computed tomography, source reconstruction in acoustics, or calculating the ...

References

Corona Australis Coronae Australis, V691 Eclipsing binaries Neutron stars {{var-star-stub