Low-mass X-ray binary

Lore & Background

LMXBs begin with a high-mass primary star and a low-mass secondary star. After the primary explodes in a supernova, the system may receive a velocity kick, often resulting in LMXBs moving at atypically fast speeds or located far from their original galactic position. These kicks are generally larger for neutron star LMXBs than for black hole LMXBs. The secondary star eventually fills its Roche lobe due to stellar evolution or loss of angular momentum, forming a common envelope and allowing accretion onto the compact object. Accretion can also occur without Roche lobe overflow, such as from a stellar core that has lost its outer layers or from stellar winds.

LMXBs are usually old systems, sometimes associated with globular clusters or the Galactic Center. Older neutron star LMXBs tend to have lower magnetic fields, typically on the order of 10^8 to 10^9 Gauss. The donor stars are generally main-sequence stars, but giant, subgiant, and white dwarf donors are also possible. In systems with M- and K-type donors (less than about 0.8 solar masses), orbital decay via magnetic braking and gravitational radiation drives the evolution, eventually leading the donor to become a brown dwarf. In systems with G-, F-, A-, and late B-type donors, stellar evolution plays a larger role, and mass transfer can lead to divergent or convergent evolution, potentially forming ultracompact X-ray binaries or LMXBs with degenerate hydrogen-rich donors.

Reader's Guide

Low-mass X-ray binaries are significant as laboratories for studying accretion physics, compact objects, and binary evolution. They produce bright X-ray emission from the release of gravitational potential energy as matter falls onto a neutron star or black hole, converting about 10% of the accreted material's rest mass into light. Neutron star LMXBs can exhibit kilohertz quasi-periodic oscillations, the fastest variability observed from any astronomical object, at frequencies of 200–1300 hertz. About one fifth of discovered neutron star LMXBs are accreting millisecond X-ray pulsars, which are believed to be the progenitors of millisecond radio pulsars. LMXBs are classified into subtypes such as Z sources (persistent, high accretion rate, tracing a Z-shape on color-color diagrams) and atoll sources (lower accretion rate, moving between banana and island states). These systems have been observed both in the Milky Way and in other galaxies, with over 200 discovered so far. Their study provides insight into the final stages of binary evolution, the behavior of degenerate matter, and the mechanisms of mass transfer and angular momentum loss in close binaries.

Did You Know?

• Over 200 LMXBs have been discovered in the Milky Way and other galaxies.
• LMXBs can have orbital periods from less than 1 hour to more than 1 year.