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X-ray binary
X-ray binary
X-ray binary
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Artist's impression of an X-ray Binary

X-ray binaries are a class of binary stars that are luminous in X-rays. The X-rays are produced by matter falling from one component, called the donor (usually a relatively common main sequence star), to the other component, called the accretor, which can be a white dwarf, neutron star or black hole. The infalling matter releases gravitational potential energy, up to 30 percent of its rest mass, as X-rays. (Hydrogen fusion 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 1041 positrons escape per second from a typical low-mass X-ray binary.[2][3]

Classification

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Microquasar SS-433.[4]

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 binary

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Artist's impression of an X-ray binary system

A low-mass X-ray binary (LMXB) is a binary star system where one of the components is either a black hole or neutron star.[1] The other component, a donor, usually fills its Roche 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 the main sequence, a degenerate dwarf (white dwarf), or an evolved star (red giant). Approximately two hundred LMXBs have been detected in the Milky Way,[11] and of these, thirteen LMXBs have been discovered in globular clusters. The Chandra X-ray Observatory has revealed LMXBs in many distant galaxies. [12]

A typical low-mass X-ray binary emits almost all of its radiation in X-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. The apparent magnitude is typically around 15 to 20. The brightest part of the system is the accretion 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 as X-ray bursters, but can sometimes be seen in the form of X-ray pulsars. The X-ray bursters are created by thermonuclear explosions created by the accretion of Hydrogen and Helium.[13]

Intermediate-mass X-ray binary

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An intermediate-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.

High-mass X-ray binary

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A high-mass X-ray binary (HMXB) is a binary star system that is strong in X rays, and in which the normal stellar component is a massive star: usually an O or B star, a blue supergiant, or in some cases, a red supergiant or a Wolf–Rayet star. The compact, X-ray emitting, component is a neutron star or black hole.[1] A fraction of the stellar wind of the massive normal star is captured by the compact object, and produces X-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 is Cygnus X-1, which was the first identified black hole candidate. Other HMXBs include Vela X-1 (not to be confused with Vela X), and 4U 1700−37.

The variability of HMXBs are observed in the form of X-ray pulsars and not X-ray bursters. These X-ray pulsars are due to the accretion of matter magnetically funneled into the poles of the compact companion.[13] The stellar wind and Roche 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 single red giant with a neutron core or a single neutron star. With a longer periodicity, a year and beyond, the HMXB can become a double neutron star binary if uninterrupted by a supernova.[14]

Be star binaries

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Be/X-ray binaries

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Be/X-ray binaries (BeXRBs) are a class of high-mass X-ray binaries that consist of a Be star and a neutron star. The neutron star is usually in a wide highly elliptical orbit around the Be star. The Be stellar 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

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ESA infographic of BeWD EP J0052 formation, observed by the Einstein Probe[16]

Be–white dwarf X-ray binary systems (BeWDs) are a rare type of X-ray binary consisting of a white dwarf that accretes matter from a rapidly-rotating Be 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.

Microquasar

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Artist's impression of the microquasar SS 433.

A microquasar (or radio emitting X-ray binary) is the smaller cousin of a quasar. 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 an accretion disk surrounding a compact object which is either a black hole or a neutron star. In quasars, the black hole is supermassive (millions of solar 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 and X-ray regions. Microquasars are sometimes called radio-jet X-ray binaries to distinguish them from other X-ray binaries. A part of the radio emission comes from relativistic jets, often showing apparent superluminal motion.[18]

Microquasars are very important for the study of relativistic 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 include SS 433, in which atomic emission lines are visible from both jets; GRS 1915+105, with an especially high jet velocity and the very bright Cygnus X-1, detected up to the High Energy gamma rays (E > 60 MeV). Extremely high energies of particles emitting in the VHE band might be explained by several mechanisms of particle acceleration (see Fermi acceleration and Centrifugal mechanism of acceleration).

See also

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References

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Further reading

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X-ray binary

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An X-ray binary is a binary star system consisting of a compact object—typically a neutron star or black hole, though sometimes a white dwarf—and a companion star from which it accretes matter, heating the infalling material to temperatures exceeding a million degrees Kelvin and producing intense X-ray emission detectable from Earth. These systems form when a massive star in a binary pair evolves and collapses into a compact remnant, while the companion continues to transfer mass through mechanisms such as Roche-lobe overflow or stellar winds, leading to the creation of an accretion disk around the compact object where X-rays are generated via thermal bremsstrahlung and other high-energy processes. X-ray binaries are classified primarily by the mass of the companion star: low-mass X-ray binaries (LMXBs) involve a low-mass companion (typically less than 1–2 solar masses), often a main-sequence star or evolved giant, with accretion primarily via Roche-lobe overflow into a stable disk; in contrast, high-mass X-ray binaries (HMXBs) feature a massive companion (greater than 8–10 solar masses), usually an O or B-type supergiant, where accretion occurs through the companion's strong stellar wind or rarely Roche-lobe overflow. A rarer intermediate-mass category exists for companions between 2 and 8 solar masses. The first X-ray binary, Scorpius X-1 (Sco X-1), was discovered in 1962 during a sounding rocket flight led by Riccardo Giacconi, marking the birth of X-ray astronomy. It is a bright, variable source about 9,000 light-years away in the constellation Scorpius. This LMXB consists of a neutron star accreting from a low-mass companion, emitting X-rays at luminosities up to 103810^{38} ergs per second—roughly 101110^{11} times brighter than the Sun in X-rays. Subsequent observations, including the 1964 detection of Cygnus X-1, identified the first strong black hole candidate in an HMXB with a 20–40 solar mass companion, demonstrating rapid X-ray variability and confirming the presence of compact objects through orbital dynamics and mass measurements. Over hundreds of known systems in the Milky Way, X-ray binaries serve as key laboratories for studying extreme astrophysics, including accretion physics, thermonuclear bursts on neutron star surfaces, pulsar timing from spinning neutron stars, and the evolution of stellar remnants, while also tracing galactic structure through their distribution—LMXBs often in globular clusters and HMXBs along spiral arms.

Overview

Definition and Characteristics

An X-ray binary is a system consisting of a —typically a or —and a normal companion star, in which from the companion to the compact object leads to the release of energy that heats accreting material to temperatures exceeding 10^7 , producing X-ray emission primarily through and non-thermal processes. These systems are distinguished from other high-energy astrophysical sources, such as active galactic nuclei, by their galactic-scale distances (typically a few kpc) and the dynamical signatures of binary orbital motion, which manifest in periodic modulations of the emission. The compact object accretes material either via Roche-lobe overflow from the companion or through stellar winds, forming an or flow that powers the X-ray output. Key characteristics of X-ray binaries include X-ray luminosities typically ranging from 10^{36} to 10^{39} erg s^{-1} in the 0.1–100 keV energy band, reflecting the efficiency of accretion onto compact objects of stellar mass. Orbital periods span a wide range, from hours in close systems to years in wider binaries, influencing the mass-transfer rate and emission stability. Spatial distributions vary by subtype: low-mass X-ray binaries (LMXBs) are preferentially found in the galactic bulge and globular clusters, while high-mass X-ray binaries (HMXBs) trace the galactic disk and spiral arms, correlating with recent star formation regions. Observationally, X-ray binaries exhibit either persistent emission or transient outbursts, with flux variability occurring on timescales from milliseconds—such as quasi-periodic oscillations (QPOs) arising from instabilities in the accretion disk—to days, driven by orbital modulation of the mass-transfer rate. Spectral features often include a soft blackbody component (kT ≈ 0.1–1 keV) from the neutron star surface or boundary layer, alongside a harder power-law continuum (photon index Γ ≈ 1.5–2.5) extending to tens of keV, resulting from Comptonization of seed photons by hot electrons in the accretion flow or corona. Basic system parameters encompass compact object masses of approximately 1.4 M_⊙ for neutron stars and 5–20 M_⊙ for black holes, determined through dynamical measurements like curves and timing. Companion star masses vary significantly by subtype, from <1 M_⊙ in LMXBs to 8–40 M_⊙ in HMXBs, affecting the evolutionary stage and accretion mode, though detailed classifications are addressed elsewhere.

Historical Discovery

The first cosmic X-ray source beyond the solar system, Scorpius X-1, was detected on June 18, 1962, by a team led by using rocket-borne proportional counters launched from , New Mexico. This unexpected discovery revealed an intense X-ray emitter in the direction of , initially leading to speculation that it might be extragalactic due to its brightness, far exceeding expectations for galactic sources. The finding, detailed in Giacconi et al.'s seminal paper, marked the birth of extragalactic X-ray astronomy and prompted further rocket experiments to map additional faint sources. The launch of the UHURU satellite (Small Astronomy Satellite-1) on December 12, 1970, ushered in the satellite era of , conducting the first all-sky survey and cataloging approximately 160 discrete sources by the end of its operations in 1973. UHURU revealed the binary nature of many sources through observations of X-ray pulsations and eclipses; for instance, Centaurus X-3 was identified as the first X-ray pulsar in 1971, with 4.8-second pulses indicating a rotating neutron star accreting from a companion. Complementary missions like Ariel 5 (launched 1974) and OSO-7 (launched 1976) expanded these findings, detecting variable and transient behaviors that solidified X-ray binaries as a distinct class powered by compact objects. Key milestones in the 1970s included the 1971 identification of as the first strong black hole candidate, based on UHURU and ground-based observations showing a massive, non-pulsing compact object in a binary system. The 1975 outburst of the transient , detected by Ariel 5 at intensities up to 50 times that of the , highlighted the episodic nature of low-mass X-ray binaries and provided early evidence for black hole accretion disks. In the 1980s and 1990s, satellites such as EXOSAT (1983–1986), Ginga (1987–1991), and ROSAT (1990–1999) refined classifications through improved timing and spectral resolution, identifying hundreds of new sources and distinguishing high- and low-mass systems via multi-wavelength correlations. The modern era, beginning with the launches of Chandra in 1999 and XMM-Newton in the same year, enabled high-resolution spectroscopy of X-ray binaries, revealing atomic lines that probe accretion environments and compact object properties. The Neutron Star Interior Composition Explorer (NICER), deployed in 2017, advanced studies of millisecond pulsars in binaries by detecting thermal X-ray pulsations from sources like PSR J0740+6620, linking spin-up mechanisms to accretion history. The eROSITA instrument on the Spektrum-Roentgen-Gamma (SRG) mission, launched in 2019, completed its first all-sky survey in 2020, with the initial data release in 2024 cataloging about 900,000 X-ray sources and providing enhanced statistics on transients and obscured objects. These developments established X-ray astronomy as a foundational field, directly advancing understandings of neutron stars, black holes, and binary evolution.

Formation and Evolution

Evolutionary Pathways

X-ray binaries form from binary star progenitors that undergo significant dynamical and evolutionary changes to produce a compact object paired with a donor star capable of transferring mass. High-mass X-ray binaries (HMXBs) typically originate from massive binary systems involving O- or B-type stars with initial primary masses exceeding 8–10 M⊙, where the primary evolves rapidly and undergoes a core-collapse supernova to form a neutron star (NS) or black hole (BH), while the secondary remains a massive companion. Low-mass X-ray binaries (LMXBs), in contrast, arise from binary progenitors where the initial primary has masses around 8–12 M⊙ (for neutron star systems) or higher for black holes, often involving a common-envelope (CE) phase where the expanding envelope of the evolving primary engulfs the low-mass secondary, leading to orbital shrinkage through angular momentum loss and eventual ejection of the envelope to form a tight orbit with a white dwarf, NS, or BH accretor. These progenitor pathways are shaped by initial separations, mass ratios, and evolutionary timescales, with massive binaries favoring HMXB formation due to their short main-sequence lifetimes of ~10 Myr. Additionally, a substantial fraction of LMXBs form dynamically in globular clusters through tidal capture or exchange interactions involving pre-existing compact objects and donor stars in these dense stellar environments. The CE ejection is particularly crucial for LMXBs to produce the compact orbits necessary for subsequent mass transfer; in these systems, this phase lasts ~10³ years and results in orbital periods of hours to days. HMXBs often form without a CE phase, maintaining wider separations suitable for wind accretion. Key evolutionary phases include the supernova kick imparted to the newborn compact object, which can reach velocities up to 400 km/s and disrupt wide binaries while tightening or eccentricizing closer orbits. Mass transfer initiates the X-ray phase once the donor fills its Roche lobe or loses material via winds, with post-supernova dynamics determining survival: kicks below ~100–200 km/s preserve most bound systems. Evolutionary tracks differ markedly by donor mass. In LMXBs, the track begins with a ~8–10 M⊙ primary evolving into an NS via supernova, paired with a low-mass (<1 M⊙) donor; angular momentum loss through magnetic braking (for convective donors) and gravitational radiation drives orbital shrinkage over ~10⁸ years, leading to stable or intermittent Roche-lobe overflow. HMXBs follow a brief track from massive primaries (~20–40 M⊙) to NS/BH with OB donors, featuring wind accretion or Roche-lobe overflow in a short-lived phase of ~10⁵–10⁶ years before the donor's own supernova disrupts or evolves the system further. Metallicity influences these tracks in HMXBs, with higher metallicity enhancing line-driven winds for wind-fed accretion, while lower metallicity promotes Roche-lobe overflow by reducing wind mass loss. The X-ray luminous phase endures ~10⁸ years in LMXBs, allowing spin-up of NSs to millisecond periods via accretion torques, often ending in detached binaries that merge via gravitational waves detectable by LIGO/Virgo. HMXB phases are shorter (~10⁵–10⁶ years), terminating in double compact objects or mergers, with limited time for extensive recycling. Population synthesis models predict approximately 40 persistent LMXBs and a total population of ~2×10³ LMXBs including transients in the Galactic bulge, consistent with Galaxy-wide estimates of hundreds of systems. These models highlight the rarity of surviving tight binaries post-supernova and the role of initial conditions in matching observed distributions.

Binary Interaction Mechanisms

In X-ray binaries, mass transfer from the donor star to the compact object occurs primarily through two modes: Roche-lobe overflow (RLOF) and stellar wind accretion. RLOF dominates in low-mass X-ray binaries (LMXBs), where the donor fills its and transfers mass steadily via an accretion disk, sustaining persistent X-ray emission; however, in soft X-ray transients, this process is unstable, leading to episodic outbursts due to temporary disk accumulation. In high-mass X-ray binaries (HMXBs), stellar wind accretion prevails, as the massive donor's strong wind envelops the compact object; the Bondi-Hoyle capture radius, defining the accretion zone, is typically around 101110^{11} cm for typical wind velocities and separations. The mass transfer rate M˙\dot{M} in RLOF systems is approximated as M˙≈(dRLdt)MdonortKH\dot{M} \approx \left( \frac{dR_L}{dt} \right) \frac{M_\mathrm{donor}}{t_\mathrm{KH}}, where RLR_L is the Roche lobe radius, MdonorM_\mathrm{donor} is the donor mass, and tKHt_\mathrm{KH} is the Kelvin-Helmholtz timescale (∼107\sim 10^7 years for low-mass donors), reflecting the donor's response to lobe shrinkage. Orbital evolution during the active phase is driven by angular momentum loss mechanisms, including gravitational waves for short-period systems (Porb∼P_\mathrm{orb} \sim hours), which cause rapid orbital decay; magnetic braking for longer periods (Porb≳1P_\mathrm{orb} \gtrsim 1 day), spinning down the donor and tightening the orbit; and isotropic re-emission from the accretion disk, ejecting specific angular momentum. The orbital separation aa evolves as dadt∝−M˙(1−β)aMtotal\frac{da}{dt} \propto -\dot{M} (1 - \beta) \frac{a}{M_\mathrm{total}}, where β\beta is the mass transfer efficiency (typically 0<β<10 < \beta < 1), indicating contraction if β<1\beta < 1. Accretion disks in these systems are prone to thermal-viscous instabilities, where partial ionization of hydrogen leads to sudden viscosity changes, triggering dwarf nova-like outbursts in soft X-ray transients by ionizing the disk and enabling rapid mass inflow. For neutron star accretors, magnetospheric interactions introduce the propeller effect: when the star's spin is rapid, the co-rotation radius exceeds the Alfvén radius rA≈(μ42GMM˙2)1/7r_A \approx \left( \frac{\mu^4}{2 G M \dot{M}^2} \right)^{1/7}, where μ\mu is the magnetic moment, MM the neutron star mass, and M˙\dot{M} the accretion rate; this expels incoming matter via centrifugal forces, suppressing accretion until spin equilibrium is reached. Typical rA∼108r_A \sim 10^8 cm for M˙∼1017\dot{M} \sim 10^{17} g s−1^{-1} and μ∼1030\mu \sim 10^{30} G cm³, highlighting the magnetic field's dominance over gas pressure in channeling flow.

System Components and Emission Processes

Compact Objects

In X-ray binaries, the compact primary object is typically either a neutron star or a stellar-mass black hole, each exhibiting distinct physical properties that influence the system's X-ray emission and dynamics. Neutron stars form through the core-collapse supernovae of massive progenitor stars with initial masses exceeding about 8 solar masses (M⊙M_\odot), resulting in remnants supported against further collapse by neutron degeneracy pressure. These objects have typical masses in the range of 1.1–2.0 M⊙M_\odot, with a median around 1.4 M⊙M_\odot, and radii constrained to approximately 10–14 km, reflecting the stiffness of the nuclear equation of state at extreme densities. Neutron stars in these systems possess strong magnetic fields, spanning 10810^8–101210^{12} gauss (G), where higher fields (∼1012\sim 10^{12} G) are common in young, high-mass X-ray binaries and lower fields (∼108\sim 10^8–10910^9 G) characterize recycled millisecond pulsars in low-mass systems due to field decay from prolonged accretion. Their spin periods vary widely: young neutron stars rotate with periods of seconds, while those in low-mass X-ray binaries can achieve millisecond spins through accretion-induced "recycling," accelerating to periods as short as 1–2 ms. Stellar-mass black holes, by contrast, arise from the final stages of massive stars (initial masses ≳20\gtrsim 20 M⊙M_\odot) via failed supernovae or direct collapse when the iron core exceeds the Tolman-Oppenheimer-Volkoff limit, avoiding a successful explosion and forming a singularity enveloped by an event horizon. In X-ray binaries, these black holes have masses generally between 3 and 100 M⊙M_\odot, though dynamically confirmed examples cluster around 5–15 M⊙M_\odot, distinguishing them from neutron stars by exceeding the maximum neutron star mass. The event horizon marks the inescapable boundary, with a Schwarzschild radius of rs=2GM/c2≈3r_s = 2GM/c^2 \approx 3 km (M/M⊙)(M/M_\odot), while the innermost stable circular orbit (ISCO) for non-spinning (Schwarzschild) black holes lies at 6GM/c2≈96GM/c^2 \approx 9 km (M/M⊙)(M/M_\odot), setting the inner boundary for stable accretion flows and influencing disk truncation and emission efficiency. Spinning (Kerr) black holes can have ISCO radii as small as GM/c2GM/c^2 for maximal prograde spin (a∗=1a^* = 1), allowing deeper accretion and higher luminosities. Distinguishing neutron stars from black holes in X-ray binaries relies on observational signatures tied to their structures. For neutron stars, coherent X-ray pulsations at frequencies of 1–1000 Hz arise from rotation-modulated emission hotspots on the magnetized surface, directly revealing the object's spin and magnetic field. Thermonuclear (type I) X-ray bursts, triggered by unstable hydrogen/helium ignition on the surface, further confirm neutron stars with burst energies ∼1039\sim 10^{39} erg and peak luminosities approaching the Eddington limit. Cyclotron resonance scattering features in the X-ray spectrum provide a direct probe of magnetic fields, with the line energy given by Ecyc=11.6 keV×B12E_\mathrm{cyc} = 11.6 \, \mathrm{keV} \times B_{12}, where B12B_{12} is the field strength in units of 101210^{12} G. Black holes, lacking a solid surface or intrinsic magnetic field, show no such pulsations, bursts, or cyclotron lines; instead, their presence is inferred from high luminosities exceeding the neutron star Eddington limit, L>LEdd≈1.4×1038(M/M⊙) erg/sL > L_\mathrm{Edd} \approx 1.4 \times 10^{38} (M/M_\odot) \, \mathrm{erg/s}, where sustained super-Eddington accretion is possible without surface effects. Accretion onto these compact objects imparts significant spin evolution. For neutron stars, the material torque drives spin-up, approximated as τ≈M˙GMrm\tau \approx \dot{M} \sqrt{G M r_m}
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