Low-mass X-ray binary

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 degeneratehydrogen or heliumcore. 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.