X-ray binaries are a class ofbinary stars that are luminous inX-rays. The X-rays are produced by matter falling from one component, called thedonor (usually a relatively commonmain sequencestar), to the other component, called theaccretor, which can be awhite dwarf,neutron star orblack hole. The infalling matter releasesgravitational potential energy, up to 30 percent of its rest mass, as X-rays. (Hydrogenfusion releases only about 0.7 percent of rest mass.) The lifetime and the mass-transfer rate in an X-ray binary depends on the evolutionary status of the donor star, the mass ratio between the stellar components, and their orbital separation.^[1]

An estimated 10⁴¹positrons escape per second from a typicallow-mass X-ray binary.^[2][3]

X-ray binaries are further subdivided into several (sometimes overlapping) subclasses, that perhaps reflect the underlying physics better. Note that the classification by mass (high, intermediate, low) refers to the optically visible donor, not to the compact X-ray emitting accretor.

• Low-mass X-ray binaries (LMXBs)
  • Soft X-ray transients (SXTs)
  • Symbiotic X-ray binaries
  • Super soft X-ray sources or Super soft sources^[5] (SSXs), (SSXB)
  • Accreting millisecond X-ray pulsars (AMXPs)^[6][7]
• Intermediate-mass X-ray binaries (IMXBs)
  • Ultracompact X-ray binaries (UCXBs)^[8]
• High-mass X-ray binaries (HMXBs)
  • Be/X-ray binaries (BeXRBs)
  • Supergiant X-ray binaries (SGXBs)
  • Supergiant Fast X-ray Transients (SFXTs)^[9][10]
• Others
  • X-ray bursters
  • X-ray pulsars
  • Microquasars (radio-jet X-ray binaries that can house either a neutron star or a black hole)

Alow-mass X-ray binary (LMXB) is abinary star system where one of the components is either ablack hole orneutron star.^[1] The other component, a donor, usually fills itsRoche lobe and therefore transfers mass to the compact star. In LMXB systems the donor is less massive than the compact object, and can be on themain sequence, a degenerate dwarf (white dwarf), or an evolved star (red giant). Approximately two hundred LMXBs have been detected in theMilky Way,^[11] and of these, thirteen LMXBs have been discovered inglobular clusters. TheChandra X-ray Observatory has revealed LMXBs in many distant galaxies.^[12]

A typical low-mass X-ray binary emits almost all of itsradiation inX-rays, and typically less than one percent in visible light, so they are among the brightest objects in the X-ray sky, but relatively faint in visible light. Theapparent magnitude is typically around 15 to 20. The brightest part of the system is theaccretion disk around the compact object. The orbital periods of LMXBs range from ten minutes to hundreds of days.

The variability of LMXBs are most commonly observed asX-ray bursters, but can sometimes be seen in the form ofX-ray pulsars. TheX-ray bursters are created bythermonuclear explosions created by the accretion of Hydrogen and Helium.^[13]

Anintermediate-mass X-ray binary (IMXB) is a binary star system where one of the components is a neutron star or a black hole. The other component is an intermediate-mass star.^[13][14] An intermediate-mass X-ray binary is the origin for Low-mass X-ray binary systems.

Ahigh-mass X-ray binary (HMXB) is abinary star system that is strong in X rays, and in which the normal stellar component is a massivestar: usually an O or B star, a bluesupergiant, or in some cases, a red supergiant or aWolf–Rayet star. The compact, X-ray emitting, component is aneutron star orblack hole.^[1]A fraction of thestellar wind of the massive normal star is captured by the compact object, and producesX-rays as it falls onto the compact object.

In a high-mass X-ray binary, the massive star dominates the emission of optical light, while the compact object is the dominant source of X-rays. The massive stars are very luminous and therefore easily detected. One of the most famous high-mass X-ray binaries isCygnus X-1, which was the first identified black hole candidate. Other HMXBs includeVela X-1 (not to be confused withVela X), and4U 1700-37.

The variability of HMXBs are observed in the form ofX-ray pulsars and notX-ray bursters. TheseX-ray pulsars are due to the accretion of matter magnetically funneled into the poles of the compact companion.^[13] Thestellar wind andRoche lobe overflow of the massive normal star accretes in such large quantities, the transfer is very unstable and creates a short lived mass transfer.

Once a HMXB has reached its end, if the periodicity of the binary was less than a year, it can become a singlered giant with a neutron core or a singleneutron star. With a longer periodicity, a year and beyond, the HMXB can become a doubleneutron star binary if uninterrupted by asupernova.^[14]

Be/X-ray binaries (BeXRBs) are a class of high-mass X-ray binaries that consist of aBe star and aneutron star. The neutron star is usually in a wide highly elliptical orbit around the Be star. The Bestellar wind forms a disk confined to a plane often different from the orbital plane of the neutron star. When the neutron star passes through the Be disk, it accretes a large mass of hot gas in a short time. As the gas falls onto the neutron star, a bright flare in hard X-rays is seen.^[15]

Be–white dwarf X-ray binary systems (BeWDs) are a rare type of X-ray binary consisting of awhite dwarf that accretes matter from a rapidly-rotatingBe star. These systems form through binary evolution where mass transfer spins up the accretor to become a Be star while the donor evolves into a white dwarf.^[17] Only eight BeWDs are known, though theoretical models say they should be 7 times more common than Be/neutron star binaries.

Amicroquasar (or radio emitting X-ray binary) is the smaller cousin of aquasar. Microquasars are named after quasars, as they have some common characteristics: strong and variable radio emission, often resolvable as a pair of radio jets, and anaccretion disk surrounding acompact object which is either ablack hole or aneutron star. In quasars, the black hole is supermassive (millions ofsolar masses); in microquasars, the mass of the compact object is only a few solar masses. In microquasars, the accreted mass comes from a normal star, and the accretion disk is very luminous in the optical andX-ray regions. Microquasars are sometimes calledradio-jet X-ray binaries to distinguish them from other X-ray binaries. A part of the radio emission comes fromrelativistic jets, often showing apparentsuperluminal motion.^[18]

Microquasars are very important for the study ofrelativistic jets. The jets are formed close to the compact object, and timescales near the compact object are proportional to the mass of the compact object. Therefore, ordinary quasars take centuries to go through variations a microquasar experiences in one day.

Noteworthy microquasars includeSS 433, in which atomic emission lines are visible from both jets;GRS 1915+105, with an especially high jet velocity and the very brightCygnus X-1, detected up to the High Energygamma rays (E > 60 MeV). Extremely high energies of particles emitting in the VHE band might be explained by several mechanisms of particle acceleration (seeFermi acceleration andCentrifugal mechanism of acceleration).

• 4U 0614+091
• LS I +61 303
• SS 433
• Quasar

• Postnov, Konstantin A.; Yungelson, Lev R. (5 May 2014)."The Evolution of Compact Binary Star Systems".Living Reviews in Relativity.17.
• Negueruela, Ignacio; Torrejon, Jose Miguel; Reig, Pablo; Ribo, Marc; Smith, David M. (2008).Supergiant Fast X-ray Transients and Other Wind Accretors. A Population Explosion: The Nature & Evolution of X-Ray Binaries in Diverse Environments. Vol. 1010. pp. 252–256.arXiv:0801.3863.Bibcode:2008AIPC.1010..252N.doi:10.1063/1.2945052.S2CID 18941968.doi:10.1063/1.2945052Bibcode:2008AIPC.1010..252N

• Audio Cain/Gay (2009)Astronomy Cast episode 135: X-ray Astronomy
• Ultraluminous X-ray Pulsar (ULXP) Catalogue