Xray Binary

X-ray binaries are a class of binary star systems distinguished by their intense emission of X-rays. This radiation is generated when matter is transferred from one stellar component, known as the donor, to a compact companion, known as the accretor. The accretor is typically a white dwarf, neutron star, or black hole. As the infalling matter accelerates under strong gravitational fields, a substantial fraction of its rest mass energy is converted into high-energy radiation, making X-ray binaries among the most energetic stellar systems known.

Physical Mechanism of X-ray Emission

The defining feature of X-ray binaries is the conversion of gravitational potential energy into radiation through accretion. Material stripped from the donor star forms an accretion disc around the compact object, where viscous processes heat the gas to temperatures of millions of kelvin. In such conditions, thermal and non-thermal processes emit strongly in the X-ray band.
The efficiency of energy release through accretion can reach up to 30 per cent of the rest mass energy of the infalling material, particularly in systems hosting black holes or neutron stars. This is significantly higher than the efficiency of hydrogen nuclear fusion, which converts only about 0.7 per cent of rest mass into energy. Consequently, accretion-powered systems can outshine nuclear-powered stars despite involving far less mass.
The lifetime of an X-ray binary and its mass-transfer rate depend on several factors, including the evolutionary state of the donor star, the mass ratio between the two components, and the orbital separation. Typical systems are also considered potential sources of antimatter, with estimates suggesting that around 10⁴¹ positrons per second may escape from an average X-ray binary.

Classification of X-ray Binaries

X-ray binaries are divided into several subclasses that reflect differences in stellar mass, accretion processes, and observational behaviour. Importantly, the terms low-mass, intermediate-mass, and high-mass refer to the mass of the donor star rather than the compact accretor.
Major categories include:

• Low-mass X-ray binaries (LMXBs)
• Intermediate-mass X-ray binaries (IMXBs)
• High-mass X-ray binaries (HMXBs)

Additional overlapping subclasses include ultracompact X-ray binaries, accreting millisecond X-ray pulsars, supersoft X-ray sources, and radio-emitting X-ray binaries, commonly known as microquasars.

Low-mass X-ray Binaries

A low-mass X-ray binary (LMXB) consists of a neutron star or black hole accreting matter from a donor star that is less massive than the compact object. The donor typically fills its Roche lobe, leading to a steady flow of matter through an accretion disc. Donor stars in LMXBs may be main-sequence stars, white dwarfs, or evolved red giants.
Approximately two hundred LMXBs have been identified in the Milky Way, with a notable concentration in globular clusters, where stellar densities favour close binary interactions. Observations by the Chandra X-ray Observatory have also revealed LMXBs in many external galaxies.
LMXBs emit the majority of their energy in X-rays, with less than one per cent radiated in visible light. Their optical faintness contrasts sharply with their extreme X-ray brightness. Typical apparent magnitudes range from 15 to 20, and the accretion disc is usually the brightest visible component.
Orbital periods vary widely, from as short as ten minutes in ultracompact systems to several hundred days. Variability is commonly observed in the form of X-ray bursts, which arise from thermonuclear explosions on the surface of accreting neutron stars caused by the accumulation of hydrogen and helium. Some LMXBs also manifest as X-ray pulsars.

Intermediate-mass X-ray Binaries

An intermediate-mass X-ray binary (IMXB) contains a neutron star or black hole accreting material from a donor star of intermediate mass. These systems are of particular evolutionary significance, as they are considered progenitors of many low-mass X-ray binaries. As mass transfer proceeds, the donor star may lose sufficient mass to evolve into a low-mass configuration, thereby transforming the system into an LMXB.
IMXBs are less commonly observed than LMXBs or HMXBs, partly due to their relatively short-lived nature and transitional evolutionary status.

High-mass X-ray Binaries

A high-mass X-ray binary (HMXB) comprises a compact object orbiting a massive donor star, typically an O-type or B-type star, a blue supergiant, or occasionally a red supergiant or Wolf–Rayet star. In these systems, X-ray emission is primarily generated when the compact object captures a fraction of the donor’s powerful stellar wind.
In contrast to LMXBs, the massive donor star dominates the optical emission, while the compact object is responsible for most of the X-rays. HMXBs are therefore easier to detect optically due to the intrinsic luminosity of the massive star.
One of the most notable HMXBs is Cygnus X-1, historically significant as the first widely accepted black hole candidate. Other prominent examples include Vela X-1 and 4U 1700−37. Variability in HMXBs is typically observed as X-ray pulsations, caused by magnetically channelled accretion onto the poles of a neutron star, rather than thermonuclear bursts.
Mass transfer in HMXBs is often unstable and short-lived. Depending on orbital parameters, the system may ultimately evolve into a single neutron star, a Thorne–Żytkow object, or a double neutron star binary if the system survives a supernova event.

Be Star X-ray Binaries

Be X-ray binaries (BeXRBs) are a prominent subclass of high-mass X-ray binaries. They consist of a neutron star in a wide, highly eccentric orbit around a rapidly rotating Be star. The Be star produces a dense equatorial disc of material that is often misaligned with the neutron star’s orbit.
When the neutron star passes through this disc, it accretes a large quantity of gas over a short time, producing intense and transient flares of hard X-rays. These episodic outbursts are a defining observational characteristic of BeXRBs.

Be–White Dwarf X-ray Binary Systems

Be–white dwarf X-ray binaries (BeWDs) are rare systems in which a white dwarf accretes matter from a Be star. They are believed to form through binary evolution involving mass transfer that spins up the Be star while the original donor evolves into a white dwarf. Only a small number of such systems are currently known, despite theoretical models predicting that they should be significantly more common than Be–neutron star binaries.

Microquasars and Radio-emitting X-ray Binaries

Microquasars are X-ray binaries that exhibit strong and variable radio emission associated with relativistic jets. These systems are considered scaled-down analogues of quasars, differing primarily in the mass of the central black hole, which in microquasars is only a few solar masses.
Microquasars contain either a black hole or neutron star accreting matter from a normal companion. The accretion disc is luminous across optical and X-ray wavelengths, while the radio emission originates from jets that may show apparent superluminal motion. Because physical timescales near a compact object scale with mass, microquasars allow the study of jet physics over humanly observable timescales.
Notable examples include SS 433, where atomic emission lines are observed from both jets, GRS 1915+105, known for its exceptionally high jet velocities, and Cygnus X-1, which has been detected up to high-energy gamma rays exceeding tens of megaelectronvolts. These systems play a crucial role in understanding particle acceleration mechanisms such as Fermi acceleration and centrifugal processes in extreme astrophysical environments.