Low-mass X-ray binaries (LMXBs or LMXRBs) are X-ray binary systems containing a compact object, typically a neutron star or black hole, and a normal star less than or equal to the mass of the Sun. LMXBs form when a high-mass star in a binary system with a low-mass star explodes in a supernova, becoming a neutron star or black hole. When the low-mass star begins to evolve, or if the binary gets closer due to orbital decay, the low-mass star fills its Roche lobe and the compact object begins to accrete off of it. Accretion from an LMXB produces bright X-ray emission that can be observed by astronomers. Over 200 LMXBs have been discovered so far, both in the Milky Way and in other galaxies. LMXBs can be classified by their properties, such as orbital period, the type of compact object present, and luminosity variability. Some types of LMXBs are exclusive to neutron star binaries, while other types can host either a neutron star or a black hole. The compact objects in LMXBs are expected to continue accreting from their companion stars until all that is left of the other star is its degenerate hydrogen or helium core. For small-enough stars, the loss of mass will be extreme enough that they will no longer be able to fuse hydrogen and will become brown dwarfs.
Formation
LMXB formation begins with a high-mass primary star (>10M☉) and a low-mass secondary star (<2M☉) in a close binary, but not so close that mass transfer occurs while both stars are still on the main sequence. When the more massive star evolves into a giant, it will fill its Roche lobe, creating a common envelope between the two stars. If the stars' orbital energy is too low, they will coalesce into a single star. However, if their orbital energy is high enough, they will spiral towards each other but not coalesce, eventually settling into a shorter but stable orbit. Eventually, the primary star will evolve into a neutron star or black hole. Later, due to stellar evolution or orbital decay, the secondary star will fill its Roche lobe and the compact object will begin to accrete off of it. A disproportionately large number of LMXBs have been observed in globular clusters. This has led some academics to speculate that interactions between stars in the globular cluster leads to the formation of LMXBs, such as tidal capture of the compact object by the companion star. However, very few black hole (BH) LMXBs are in globular clusters. Some academics have suggested that LMXBs originally start out in globular clusters before spreading out throughout the galaxy, however, this does not explain the observed abundance and distribution of BH LMXBs.
Properties Unlike HMXBs, LMXBs are typically devoid of typical stellar absorption features, possibly because the total emission from the system is dominated by light from the accretion disk of the compact object. The intrinsic luminosity of an LMXB scales with its mass accretion rate. However, the apparent (observed) luminosity of an intrinsically-bright LMXB may be low due to occultation of the source. X-ray emission is particularly strong in the inner accretion disk region.
Orbital periods LMXB orbital periods are generally short compared to the orbital periods of other types of binary stars, typically less than 1 day. When the companion star is a main-sequence star, LMXB orbital periods are typically on the order of hours, while for LMXBs with giant companions, orbits can last days to weeks. Additionally, LMXBs with the lowest accretion rates tend to have shorter orbital periods than LMXBs with near-Eddington accretion rates, particularly in black hole systems. It is likely that many of these near-Eddington LMXBs host a giant donor star. Confirmed BH LMXBs all have orbital periods ranging from hours to weeks; there is a notable paucity with periods less than 4 hours or greater than 1 week. Conversely, neutron star (NS) LMXBs have much wider variation in orbital duration, with periods of confirmed NS LMXBs ranging from 11 minutes to 3.1 years. Additionally, unlike BH LMXBs, many NS LMXBs have been observed with very short orbital periods, on the scale of a few hours or less.
Dynamics Most LMXBs do not accrete continuously but instead go through periods of activity (outbursts) before settling back into quiescence; such LMXBs are called transients. Outbursts in transients generally lasts for weeks to months at a time, while quiescent periods of little-to-no accretion can last for months to decades. LMXBs that are not transients (are continuously accreting) are called persistent LMXBs. Persistent LMXBs may still vary in their accretion rate over time, but it is not as extreme as transient LMXBs. Quasi-persistent LMXBs are LMXBs that exhibit long outbursts (>1 year at a time) but still have periods of quiescence. Accretion onto most LMXBs comes from Roche lobe overflow. The secondary star will start to fill its Roche lobe, either due to stellar evolution or loss of angular momentum in the system, and will form a common envelope with the primary object. Mass from the secondary star will then accrete onto the compact object. However, it is also possible for accretion to take place without a star having filled its Roche lobe; for example, a compact object may accrete off of a stellar core which has lost its outer layers or from material ejected from the secondary star by stellar winds. LMXB donor stars are generally main-sequence stars. However, giant, subgiant, and white dwarf donors are also possible.
Evolution LMXBs are usually old systems, sometimes associated with regions of older star populations, such as globular clusters or the Galactic Center. Older NS LMXBs tend to have lower NS magnetic fields, as most of the NS's original magnetic field has dissipated since forming; these LMXBs typically have magnetic field strengths to the order of 108–1010 Gauss. When the primary object in an LMXB undergoes a supernova, it can generate a significant velocity kick to the system, resulting in many observed LMXBs moving at atypically fast speeds relative to Earth or located far from their original position in the galaxy. These kicks are generally larger for NS LMXBs than for BH LMXBs.
In systems with M- and K- type donor stars In LMXBs where the donor star is an M- or K-type star–less than about 0.8M☉–the donor is not yet old enough to fill its Roche lobe due to stellar evolution given the age of the universe, so sufficient orbital decay must happen in order for the star to fill its Roche lobe. This will first happen via magnetic braking, which causes the orbital period of the binary to decrease. The compact object will begin accreting off of the star, and, to maintain stability, the star will swell in size. Eventually, however, the star will become fully convective, magnetic braking will cease, and the star will shrink, causing it to no longer fill its Roche lobe. At this point, the system is a detached binary with a period of about 3 hours. The orbit of the two objects will continue to decay due to gravitational radiation, albeit significantly slower than before, until the donor star fills its Roche lobe again. The compact object will once again accrete matter from the donor star, which will eventually lose enough mass to be unable to sustain hydrogen fusion and therefore become a brown dwarf. Once the donor is a brown dwarf, it will expand in response to mass loss, causing the orbital period to increase again.
In systems with G-, F-, A-, and late B-type donor stars In LMXBs where the donor star is a main-sequence star more massive than K-type (stars of spectral class G or earlier), stellar evolution of the donor star will generally influence the evolution of the system. For systems with radiative common envelopes, mass transfer is triggered only by stellar evolution of the donor star. When stable mass transfer begins, the orbital period of the binary will increase in response to mass loss from the donor star. For stars with convective common envelopes, mass transfer is triggered by both magnetic braking and stellar evolution. When stable mass transfer begins, these binaries can evolve in one of two ways. If the nuclear timescale of the donor star is greater than the timescale of mass loss due to magnetic braking, the evolution of the binary will be based on the donor star's stellar evolution, and the orbital period of the binary will increase with mass loss. Otherwise, the evolution of the binary will be dominated by magnetic braking, and the binary's orbit will decay. These two paths are known as divergent and convergent evolution, respectively. In convergent systems, if the donor star has enough time to evolve a degenerate helium core, the system will end up as a ultracompact X-ray binary (UCXB) with a very short orbital period and a helium-rich degenerate donor star. Otherwise, the system will evolve into an LMXB with a hydrogen-rich degenerate donor star. In cases where the donor star is more massive than the compact object, mass transfer may initially be thermally unstable, driving the donor star out of thermal equilibrium; however, eventually, mass transfer will stabilize and thermal equilibrium will be restored.
Subtypes
Neutron star LMXBs Neutron star (NS) LMXBs primarily generate energy through the release of gravitational potential energy, converting about 10% of the accreted material's rest mass-energy into light, mostly in X-rays. NS LMXBs can sometimes exhibit a very fast type of quasi-periodic oscillation known as kilohertz QPOs. At 200–1300 hertz, depending on the state of the NS, these QPOs are the fastest variability observed from any astronomical object.
Accreting millisecond X-ray pulsars
About one fifth of discovered neutron star LMXBs can be classified as accreting millisecond X-ray pulsars (AMXPs), sometimes called accreting millisecond pulsars (AMSPs). All known AMXPs are transients, and tend to emit in hard (high-energy) X-rays during their outbursts. The accretion flow in these systems is shaped by the neutron star's magnetic field, pulling material towards the neutron star's magnetic poles. This phenomenon increases the spin rate of the neutron star and generates X-ray pulsations every 1–10 milliseconds as the pulsar rotates. Companion stars that have been observed in AMXPs include brown dwarfs, white dwarfs, helium stars, and main-sequence stars. AMXPs are believed to be the progenitors of millisecond radio pulsars.
Z sources
Z sources are persistent NS LMXBs with high, near-Eddington accretion rates. They trace a Z-shape when their luminosity is plotted on an X-ray color-color diagram. Jets have been observed in Z sources, and Z sources tend to be brighter than atoll sources in both X-ray and radio wavelengths. The three branches of the Z are called the horizontal branch, normal branch, and flaring branch. A Z source moves along the Z-track over periods of hours to days.
Atoll sources
Atoll sources are NS LMXBs with accretion rates that are relatively low compared to the Eddington limit. They can be persistent or transient. Atoll sources move between two states, the banana state and the island state, named after their shapes on an X-ray color-color diagram. They spend much longer than Z sources moving along the color-color diagram, on the order of days to weeks, spending most of that time in the island state. A subclass of atoll sources, bright atolls, are persistent LMXBs and are almost always in the banana state. Jets have been observed in some atoll sources.
Black hole LMXBs Although many BH LMXBs have been observed, they are outnumbered by NS LMXBs by a factor of approximately 2:1. This aligns with typical values for the initial mass function, which predicts much higher rates of neutron star formation than black hole formation. Like NS LMXBs, many BH LMXBs have been observed to have jets. These jets are emitted in the hard (high-energy) X-ray state, and then are extinguished in the soft (low-energy) X-ray state, when the accretion rate is low.
Ultracompact X-ray binaries
Ultracompact X-ray binaries (UCXBs) are LMXBs with very short orbital periods of approximately 80 minutes or less. They can contain either a NS or BH accretor; however, most discovered galactic UCXBs contain a NS rather than a BH. Only white dwarfs and helium-burning subgiants are small enough for these orbital periods. UCXBs can be detected by observation of periodic changes in the system associated with orbital motion, or by the rate of X-ray pulsations in the case of systems with accreting millisecond pulsars, as the pulsation rate is correlated with the system's orbital period. Unlike most other LMXB types, UCXBs are generally persistent. Their elemental composition is largely helium, carbon, oxygen, and neon, with very low abundances of hydrogen. UCXBs are believed to be major sources of gravitational waves; unlike other types of LMXBs, where orbital decay may be driven largely by magnetic braking, orbital decay in UCXBs is driven mainly by gravitational radiation.
Symbiotic X-ray binaries Unlike other classes of LMXBs, symbiotic X-ray binaries (SyXBs) do not experience Roche lobe overflow. Rather, the primary object accretes stellar wind ejected from the secondary, a late-type giant. Most SyXBs show significant variability, shifting between outburst and quiescent phases on timescales of hours or days. However, their emission patterns are very sporadic, making them frequently difficult to detect and classify. As of 2024, neutron stars have been identified as the primary object in several SyXBs, but no black hole SyXBs have been identified. However, black holes have not been ruled out as primary objects in SyXBs, and the nature of the compact object in many SyXBs is unknown.
Soft X-ray transients
Soft X-ray transients (SXTs) are a type of LMXB transients that spend most of their time in quiescence, but exhibit occasional flares with luminosities peaking around 3–200 keV. Most known SXTs are black hole binaries. When SXTs flare, they are most luminous in soft X-rays generated by thermal blackbody emission. SXTs with longer orbital periods have brighter flares. This is likely because the tendency of a binary to be transient is modulated by its orbital period, with longer orbital periods allowing for higher mass accretion rates. Accretion rates in NS SXTs and BH SXTs are similar.
Supersoft X-ray sources
Supersoft X-ray sources (SSSs or SSXSs), also known as supersoft x-ray binaries, are X-ray sources emitting soft X-rays at very high luminosities. Believed to contain a white dwarf accreting off of a low-mass star, SSXSs do not fit with traditional properties of either cataclysmic variables or LMXBs. Therefore, although SSXSs are binaries and are emitting largely in X-rays, they are not classified as LMXBs by some definitions, particularly those requiring that the accreting object in an LMXB be a neutron star or black hole. It is possible, however, that some SSXSs may actually contain neutron stars, or even black holes.
Observation As of 2024, around 200 confirmed and candidate LMXBs have been discovered in the Milky Way. When LMXBs are actively accreting, they become some of the brightest X-ray point sources in the sky, allowing for their discovery and identification. Due to advances in telescope technology, transient LMXBs are being discovered increasingly quickly. During accretion, the spectrum emitted by an LMXB is overwhelmed by emission from the accretion disk of the NS or BH, even at low-frequency wavelengths. This makes it difficult to study the properties of the companion star. Although transient LMXBs can be studied during quiescence, when accretion disk emission is low enough to discern the spectrum of the donor star, this poses a problem for persistent LMXBs, which never have optimal periods to study the companion star. Observation of the compact object during quiescence can also provide scientific insight. For example, studying blackbody radiation emitted from NSs in LMXBs can allow the radius of the NS to be estimated, providing constraints on potential neutron star equations of state.
Differentiating NS and BH LMXBs
The presence of a neutron star in an LMXB can be confirmed by identifying events that require a solid surface, which a black hole does not have. For example, observations of type I X-ray bursts can confirm a system has a neutron star, as they arise from thermonuclear runaway of accreted matter on the star's surface. These bursts are very energetic, emitting 1039 ergs of energy over just a few seconds. X-ray pulsations can also confirm that an LMXB contains a neutron star, since they occur due to hotspots on the surface of the star. However, not all NS LMXBs exhibit pulsations or bursts, so the presence of one should not be ruled out if these phenomena are not detected. In fact, identifying the compact object in LMXBs with very low luminosities can be a challenge due to the typical absence of these features even in NS systems. NS and BH LMXBs can also be differentiated by emission from their jets. Both NS and BH LMXBs can have jets, but for similar accretion rates, BH LMXBs have much more radio emission from jets than NS LMXBs, typically by a factor of 5–20. However, levels of radio emission can overlap, so LMXBs with high radio emission can host NSs, and LMXBs with low radio emission can contain BHs. Blackbody emission in X-rays from quiescent LMXBs can confirm that the system contains a neutron star. The surface of a NS is typically around 106 K, which is high enough to peak in soft X-rays and be easily distinguishable from the blackbody spectrum of a companion star with a much cooler surface. However, this method is only applicable to transients, and extinction can prevent the observation of soft X-ray radiation. Additionally, a sufficiently cool neutron star may peak outside of the X-ray range. Thus, the lack of an observed NS blackbody spectrum cannot rule out the presence of a NS in an LMXB. Estimation of the mass of the compact object can directly differentiate NS and BH LMXBs, as NSs >3M☉ would be unable to support themselves via neutron degeneracy pressure and would collapse into black holes. Mass estimates typically rely on radial velocity spectroscopy, which measures the shifts in the spectrum of the visible companion star as it moves in orbit with the compact object. However, the angle of inclination of the binary's orbit with respect to Earth is needed to accurately pinpoint the mass of the compact object, and this angle is often difficult to determine observationally. However, even with an unknown angle of inclination, a lower limit on the mass of the compact object can be placed based on radial velocity alone; if this mass is greater than 3M☉, the compact object in the system can be confidently identified as a black hole.
See also Blue straggler X-ray pulsar Variable star Stellar vampirism
Notes
References