X-ray binary

X-ray binaries are powered by mass transfer onto a compact stellar remnant. The compact object is generally a neutron star or black hole, while the donor is a normal star, a giant, or a stripped star. Gas captured by gravity usually forms an accretion disk; friction and magnetic stresses convert orbital energy into heat and radiation.¹

The donor can lose material through a Roche lobe when it expands to fill the gravitational region around it, or through a fast stellar wind. Matter spiraling inward emits primarily X-rays, although optical, ultraviolet, radio, and sometimes gamma-ray signals can accompany the system. Orbital motion, eclipses, and changes in brightness allow astronomers to estimate masses that cannot be measured directly.

The two broad classes are low-mass and high-mass X-ray binaries, distinguished chiefly by the mass and evolutionary state of the donor star.² Low-mass systems contain donors of roughly one solar mass or less and commonly transfer matter through Roche-lobe overflow. Their long-lived evolution can spin up a neutron star into a millisecond pulsar after the donor has supplied angular momentum.

High-mass X-ray binaries have young, luminous O- or B-type companions. They are often fed by stellar winds, although some Be-star systems transfer gas from a transient equatorial disk. Because massive stars evolve quickly, these binaries trace recent star formation. A supernova can leave a neutron star or black hole in orbit, but asymmetric explosions may also disrupt the binary or give the remnant a substantial natal kick.³

Variability is the main observational signature of an X-ray binary. Systems may brighten by orders of magnitude during an outburst, fade into quiescence, or show rapid fluctuations from the inner disk and compact-object surface. In neutron-star systems, thermonuclear fuel accumulated on the surface can ignite in a Type I X-ray burst, lasting seconds to minutes and sometimes recurring over hours.

Black-hole binaries often display changing spectral states, including a disk-dominated state and a harder state associated with a hot corona or relativistic jet. Pulsations reveal a rotating, magnetized neutron star, while eclipses and Doppler shifts constrain orbital geometry and component masses. Space observatories such as Chandra, XMM-Newton, NuSTAR, and NICER combine imaging, spectroscopy, and high-speed timing to distinguish these processes.⁴

Some X-ray binaries are transient because an unstable disk alternately stores and releases matter. In the disk-instability cycle, hydrogen ionization changes the disk’s viscosity, producing recurrent outbursts rather than steady emission. This behavior makes otherwise faint systems detectable across the Galaxy.

Not every compact-object binary is a bright X-ray source: a detached pair may remain nearly invisible until wind capture, Roche-lobe overflow, or a common-envelope phase changes the interaction. Conversely, an apparently ordinary companion can conceal a black hole identified through orbital motion alone. X-ray binaries also connect stellar astrophysics with broader questions, including the origin of millisecond pulsars, the demographics of stellar-mass black holes, jet formation, and the possible progenitors of some gravitational-wave events.

Accretion disk

A rotating disk of gas whose gravitational energy is converted into heat and radiation as the gas moves inward.

Roche lobe

The region around a star in a binary within which material remains gravitationally bound to that star.

Type I X-ray burst

A thermonuclear flash on the surface of an accreting neutron star.

Quiescence

A relatively faint interval between major outbursts of a transient X-ray binary.