An Etymological Dictionary of Astronomy and Astrophysics
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A member of one of the two main classes of → X-ray binary systems where one of the components is a neutron star or a black hole and the other one a → massive star. HMXBs emit relatively → hard X-rays and usually show regular pulsations, no X-ray bursts, and often X-ray eclipses. Their X-ray luminosity is much larger than their optical luminosity. In our Galaxy HMXBs are found predominantly in the → spiral arms and within the → Galactic disk in young → stellar populations less than 10⁷ years old. One of the most famous HMXB is Cygnus X-1 which was the first stellar-mass black hole discovered. See also: → low-mass X-ray binary.

→ high; → mass; → X-ray; → binary.

A member of one of the two main classes of → X-ray binary systems where one of the components is a → neutron star or → black hole and the other component a → low-mass star with a spectral type A or later. LMXBs mainly emit → soft X-rays. The ratio of their optical to X-ray luminosities is less than 0.1. They belong to → old stellar populations with ages 5-15 × 10⁹ years and are found in → globular clusters and in the → bulge of our → Milky Way galaxy; some are also found in the disk. Hercules X-1 is an example of LMXBs.
See also: → high-mass X-ray binary.

→ low; → mass; → X-ray; → binary.

A binary star system where one of the stars has evolved and collapsed into an extremely dense body such as a → white dwarf, a → neutron star, or a → black hole. The enormous gravitational attraction of the massive, dense, but dim component pulls material from the brighter, less massive star in an → accretion disk. The gravitational potential energy of the accreted matter is converted to heat by → viscosity and eventually to high-energy photons in the X-ray range. The brightest X-ray binary is → Scorpius X-1.

→ X-ray; → binary.