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 (>10) and a low-mass secondary star (<2) 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 – 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 0.8–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