Low-mass X-ray binary
Binary systems where a compact object accretes from a low-mass star.
Low-mass X-ray binaries, or LMXBs, are pairs of stars where one is a compact object—either a neutron star or a black hole—and the other is a normal star no more massive than the Sun. These systems begin as a binary containing a high-mass star and a low-mass companion. When the high-mass star goes supernova, it leaves behind a neutron star or black hole. Later, as the low-mass star evolves or the binary orbit shrinks, that star spills material over its Roche lobe, and the compact object pulls it in. This accretion process generates bright X-rays that astronomers can detect.
More than 200 LMXBs have been found, both in the Milky Way and in other galaxies. They are categorized by traits like orbital period, the type of compact object, and how their brightness varies. Some types occur only with neutron stars, while others can involve either a neutron star or a black hole. Over time, the compact object keeps accreting until only the degenerate hydrogen or helium core of the companion remains. If the companion star is small enough, it loses so much mass that it can no longer fuse hydrogen and becomes a brown dwarf.
**Formation**
An LMXB starts with a massive primary star (over ten times the Sun's mass) and a low-mass secondary (under two solar masses) in a close binary—but not so close that mass transfer happens while both are still on the main sequence. When the primary swells into a giant, it fills its Roche lobe, creating a common envelope around both stars. If the orbital energy is too low, the stars merge into one. If the energy is high enough, they spiral inward without merging, settling into a tighter, stable orbit. Eventually, the primary becomes a neutron star or black hole. Later, due to stellar evolution or orbital decay, the secondary fills its Roche lobe, and accretion onto the compact object begins.
A surprisingly large number of LMXBs are found in globular clusters. Some researchers think this is because interactions between stars in these dense clusters—like tidal capture of the compact object by a companion—help form LMXBs. However, very few black hole LMXBs reside in globular clusters. Some have suggested that LMXBs start in globular clusters and then spread through the galaxy, but this idea doesn't explain the observed numbers and distribution of black hole LMXBs.
- Field
- Astronomy
- Known for
- Bright X-ray emission from accretion onto neutron stars or black holes
- Number discovered
- Over 200
- Typical orbital periods
- Less than 1 day, ranging from 11 minutes to 3.1 years
- Common locations
- Milky Way, other galaxies, globular clusters
Lore & Background
LMXBs begin with a high-mass primary star and a low-mass secondary star in a close binary. When the primary evolves into a giant, it fills its Roche lobe, creating a common envelope. If orbital energy is sufficient, the stars spiral closer without coalescing, settling into a shorter orbit. The primary eventually becomes a neutron star or black hole. Later, the secondary fills its Roche lobe due to stellar evolution or orbital decay, and accretion begins.
Reader's Guide
LMXBs are significant because they produce bright X-ray emission, allowing astronomers to study accretion processes onto neutron stars and black holes. Over 200 have been discovered in the Milky Way and other galaxies. They are classified by orbital period, compact object type, and luminosity variability. A disproportionately large number are found in globular clusters, though black hole LMXBs are rare there. LMXBs are usually old systems, often associated with older star populations. Their evolution can lead to the donor star becoming a brown dwarf or, in some cases, forming ultracompact X-ray binaries. The study of LMXBs provides insights into binary evolution, stellar death, and the behavior of matter under extreme gravity.
Did You Know?
- Over 200 LMXBs have been discovered so far, both in the Milky Way and in other galaxies.
- Confirmed neutron star LMXBs have orbital periods ranging from 11 minutes to 3.1 years.
- LMXBs are usually old systems, sometimes associated with globular clusters or the Galactic Center.
- In systems with M- and K-type donor stars, the donor may eventually become a brown dwarf.
More in Astronomical X-ray sources 1-24
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