X-ray binary

An X-ray binary is a stellar system in which a neutron star or black hole accretes matter from a companion star, heating it until it emits intense X-rays.

Accretion disk: A rotating flow of gas that spirals inward as it loses angular momentum. Transferred gas often forms a disk whose inner regions heat to X-ray-emitting temperatures.
Neutron star: A dense stellar remnant composed primarily of neutrons, formed when a massive star’s core collapses. It is one of the two compact-object types that power X-ray binaries.
Cygnus X-1: A high-mass X-ray binary containing a black hole and the massive star HDE 226868. It is a well-studied example of wind-fed accretion onto a black hole.
Cataclysmic variable: A binary system in which a white dwarf accretes matter from a companion star. Its accretor is a white dwarf, unlike the neutron stars and black holes considered here.
Roche-lobe overflow: Mass transfer that begins when a star expands beyond the gravitational region in which its matter remains bound to it. A companion can feed the compact object when it fills its Roche lobe.
Black hole: A region of spacetime from which nothing, including light, can escape beyond its event horizon. Black holes accrete companion-star matter without a solid surface.
Scorpius X-1: A bright low-mass X-ray binary containing a neutron star and a low-mass companion. Its strong X-ray emission helped establish the class observationally.
High-mass X-ray binary: An X-ray binary whose donor is a massive, early-type star. Massive donors often feed the compact object through stellar winds.
Eddington luminosity: The luminosity at which radiation pressure balances gravity for spherical accretion onto an object. It provides a benchmark for interpreting how brightly an X-ray binary accretes.
Stellar evolution: The changes in a star’s structure and properties over its lifetime. A binary’s history determines when its compact object and mass donor form.

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