X-ray binary
Binary star systems luminous in X-rays via accretion.
(Credit: NASA/CXC/M.Weiss) · Public domain
X-ray binaries are a type of binary star system that shine brightly in X-rays. This emission comes from material falling from one star—the donor, often a typical main sequence star—onto a compact companion called the accretor, which may be a white dwarf, neutron star, or black hole. As the infalling matter releases gravitational potential energy, it can convert up to 30 percent of its rest mass into X-rays (by comparison, hydrogen fusion releases only about 0.7 percent). How long the system lasts and how fast mass transfers depend on the donor star’s evolutionary stage, the mass ratio between the two stars, and how far apart they orbit. In a typical low-mass X-ray binary, the number of positrons escaping each second is highly uncertain and not a standardly established figure in the literature.
**Classification**
X-ray binaries are divided into several subclasses, some overlapping, which often better reflect the underlying physics. The classification by mass—high, intermediate, or low—refers to the optically visible donor star, not the compact X-ray-emitting accretor.
- Low-mass X-ray binaries (LMXBs) - Soft X-ray transients (SXTs) - Symbiotic X-ray binaries - Super soft X-ray sources (SSXs or SSXBs) - Accreting millisecond X-ray pulsars (AMXPs) - Ultracompact X-ray binaries (UCXBs) - Intermediate-mass X-ray binaries (IMXBs) - High-mass X-ray binaries (HMXBs) - Be/X-ray binaries (BeXRBs) - Supergiant X-ray binaries (SGXBs) - Supergiant Fast X-ray Transients (SFXTs) - Others: X-ray bursters, X-ray pulsars, microquasars (radio-jet X-ray binaries that can host a neutron star or black hole)
**Low-mass X-ray binary**
A low-mass X-ray binary (LMXB) is a binary system where one component is a black hole or neutron star. The donor usually fills its Roche lobe, transferring mass to the compact object. In LMXBs, the donor is less massive than the compact object and can be a main sequence star, a white dwarf, or an evolved red giant. About two hundred LMXBs have been found in the Milky Way, with thirteen located in globular clusters. The Chandra X-ray Observatory has detected LMXBs in many distant galaxies. A typical LMXB emits nearly all its radiation in X-rays, with less than one percent in visible light, making them among the brightest X-ray sources but faint optically—their apparent magnitude is usually around 15 to 20. The brightest part of the system is the a
- field
- Astrophysics
- known_for
- Luminous X-ray emission from accreting compact objects
- subclasses
- Low-mass, intermediate-mass, high-mass, Be/X-ray, microquasars, and others
Lore & Background
X-ray binaries are subdivided into several (sometimes overlapping) subclasses that perhaps reflect the underlying physics better. The classification by mass (high, intermediate, low) refers to the optically visible donor, not to the compact X-ray emitting accretor. Low-mass X-ray binaries (LMXBs) have a donor less massive than the compact object, which can be a main sequence star, white dwarf, or red giant. Approximately two hundred LMXBs have been detected in the Milky Way, and of these, thirteen have been discovered in globular clusters. The Chandra X-ray Observatory has revealed LMXBs in many distant galaxies. A typical LMXB emits almost all its radiation in X-rays, with less than one percent in visible light, making them among the brightest objects in the X-ray sky but relatively faint in visible light. Their orbital periods range from ten m
Reader's Guide
High-mass X-ray binaries (HMXBs) have a massive normal star (usually O or B star, blue supergiant, or in some cases red supergiant or Wolf–Rayet star) as the donor. A fraction of the stellar wind is captured by the compact object, producing X-rays. One of the most famous HMXBs is Cygnus X-1, the first identified black hole candidate. The variability of HMXBs is observed in the form of X-ray pulsars, not X-ray bursters. Be/X-ray binaries (BeXRBs) are a class of HMXBs consisting of a Be star and a neutron star in a wide highly elliptical orbit; when the neutron star passes through the Be disk, it accretes a large mass of hot gas, producing a bright flare in hard X-rays. Microquasars (or radio-jet X-ray binaries) are smaller cousins of quasars, with an accretion disk surrounding a black hole or neutron star, and show strong and variable radio emission often resolvable as a pair of relativistic jets. Noteworthy microquasars include SS 433, GRS 1915+105, and Cygnus X-1.
Did You Know?
- The infalling matter in an X-ray binary releases up to 30 percent of its rest mass as X-rays, compared to only 0.7 percent from hydrogen fusion.
- An estimated 1041 positrons escape per second from a typical low-mass X-ray binary.
- Low-mass X-ray binaries have orbital periods ranging from ten minutes to hundreds of days.
- Be–white dwarf X-ray binary systems are a rare type, with only eight known, though theoretical models say they should be 7 times more common than Be/neutron star binaries.
Gallery
Frequently Asked Questions
What exactly is an X-ray binary?
An X-ray binary is a pair of stars in which a compact object—such as a white dwarf, neutron star, or black hole—draws gas from its companion and shines brightly in X-rays as that material spirals inward.
How do X-ray binaries produce so much X-ray radiation?
As gas from the donor star falls onto the compact accretor, gravitational potential energy is released so efficiently that up to 30 percent of the infalling matter's rest mass can be converted into X-rays, far exceeding the roughly 0.7 percent yield of ordinary hydrogen fusion.
What are the main subclasses of X-ray binaries?
Astronomers divide them into low-mass, intermediate-mass, and high-mass categories based on the donor star, while special groups include Be/X-ray systems and microquasars that launch relativistic jets.
Why are X-ray binaries important in astrophysics?
They act as natural laboratories for probing accretion physics, extreme gravity, and the behavior of matter under conditions impossible to reproduce in a lab, making them central to our understanding of compact objects and high-energy processes.
How long does an X-ray binary system survive?
The system's lifetime and the pace of mass transfer depend heavily on the donor star's evolutionary stage and the mass ratio between the two stars, so some binaries burn through their fuel quickly while others persist for millions of years.
More in Astronomical X-ray sources 1-24
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