X-ray binary

X-ray binaries shine because mass transfer converts orbital energy into high-energy radiation. The compact object is paired with a normal star, and the system may be close enough for the donor to fill its Roche lobe or may capture part of a strong stellar wind. Matter forms an accretion disk in many systems; its inner regions emit X-rays, while jets can carry some energy away.

The compact object is either a neutron star or a black hole. Neutron-star binaries can display thermonuclear bursts from accumulated surface fuel, whereas black-hole binaries lack a visible material surface and are identified through their dynamics and characteristic X-ray states.² The companion ranges from a low-mass star like the Sun to a massive, short-lived O- or B-type star.

X-ray binaries are commonly divided by the mass of the donor star and by the way mass is supplied. Low-mass X-ray binaries generally contain a late-type donor and a neutron star or black hole; many are transient, switching between faint quiescence and bright outbursts. High-mass X-ray binaries contain young, luminous donors, and their compact objects often accrete from a stellar wind or from a Be-star disk.¹

Their brightness and spectrum can change on timescales from milliseconds to years. Black-hole systems often move among hard, soft, and intermediate states as the disk and hot corona change. Pulsars reveal rotating, magnetized neutron stars, while eclipses and Doppler shifts allow astronomers to measure orbital geometry and constrain compact-object masses.²

An X-ray binary usually forms after one member of a primordial binary undergoes a supernova and leaves a compact remnant. The explosion can disrupt the pair, but systems that remain bound may acquire an eccentric orbit and a moving center of mass. Later, the surviving star expands or loses mass, initiating renewed accretion.

Binary evolution can reverse the apparent age ordering: the initially more massive star becomes the compact object, while the original secondary later donates material. Accretion can spin up a neutron star into a millisecond pulsar, and repeated mass exchange can alter chemical abundances and orbital periods. Some systems are candidate progenitors of compact-object mergers, although the connection depends on uncertain mass loss, supernova kicks, and common-envelope evolution.³

Several X-ray binaries are important outside the standard bright-disk picture. In wind-fed systems, the accretion flow may be clumpy and highly variable rather than a stable disk. A neutron star's magnetic field can channel gas onto small polar regions, producing rotating X-ray pulses and sometimes cyclotron spectral features. In other systems, radiation pressure drives disk winds that remove substantial mass and angular momentum.

Transient binaries also provide a rare view of quiescent compact objects: a faint black-hole candidate may be recognized through its orbital motion even when accretion has nearly stopped. X-ray binaries occur in globular clusters, the Milky Way's disk and bulge, and nearby galaxies such as the Magellanic Clouds. Their collective emission can help trace recent star formation and the population of compact remnants in a galaxy.

Accretion disk

A rotating disk of gas spiraling toward a compact object and releasing gravitational energy as heat and radiation.

Roche lobe

The region around a star in a binary within which its gravity dominates; overflow transfers matter to the companion.

Transient

An X-ray binary whose brightness changes dramatically, usually through episodic accretion outbursts.

X-ray pulsar

A magnetized, rotating neutron star whose accretion-powered emission varies periodically as its beam sweeps past Earth.