X-ray binary

Two stars dance in space.

One star is very small. It pulls stuff from the other star. This stuff falls toward the small star. As it falls, it makes a bright light. This light is a special kind. We call it an X-ray.

The bright light comes from the falling stuff. Some stars are very big. Some stars are small. These stars can even make jets of light. It is a busy dance in the sky!

Some stars live in pairs. We call these binary stars.

In an X-ray binary, one star is very special. It is a compact object. This could be a white dwarf, a neutron star, or a black hole. The other star is called the donor.

The compact object pulls matter from the donor star. This matter falls toward the compact object. As it falls, it gives off power called X-rays. This happens because the matter loses gravitational potential energy. This process can be very bright.

There are different kinds of these systems. In low-mass X-ray binaries, the donor star is small. These can make X-ray bursters. These are bright flashes caused by tiny explosions. In high-mass X-ray binaries, the donor is a massive star. These often show X-ray pulsars. These are steady pulses of light.

Some systems are called microquasars. They are like small versions of quasars. They can make jets of gas that shoot out from the center. These jets move very fast.

Some stars travel through space in pairs. We call these binary stars.

An X-ray binary is a special kind of pair. One star is a compact object. This could be a white dwarf, a neutron star, or a black hole. The other star is called the donor. The donor star gives matter to the compact object. This matter releases a huge amount of energy. It can release up to 30 percent of its rest mass as X-rays.

This is much more than the energy from hydrogen fusion. Hydrogen fusion only releases about 0.7 percent of rest mass.

How does this light happen? It works through a step-by-step way of moving matter. The donor star loses matter to the compact object. This matter falls toward the compact star. As the matter falls, it loses gravitational potential energy. This energy turns into bright X-rays. In some systems, an accretion disk forms around the compact object. This disk is the brightest part of the system.

In low-mass systems, matter can cause X-ray bursters. These are tiny explosions made of hydrogen and helium. In high-mass systems, magnetic fields can funnel matter into poles. This creates X-ray pulsars that flash steadily.

Scientists have found many different types of these systems. They group them by the mass of the donor star. Low-mass X-ray binaries (LMXBs) have a small donor star. These donors can be a main sequence star or a red giant.

Intermediate-mass X-ray binaries (IMXBs) have a star of medium size. High-mass X-ray binaries (HMXBs) have a very large donor star. These massive stars are often blue supergiants or Wolf–Rayet stars. Some HMXBs are called Be/X-ray binaries. These involve a Be star and a neutron star in a wide orbit.

We have learned a lot about these stars over time. Astronomers have detected about two hundred LMXBs in our Milky Way. Thirteen of these are found in globular clusters. The Chandra X-ray Observatory has seen LMXBs in far-off galaxies too. Cygnus X-1 is a very famous high-mass X-ray binary. It was the first object identified as a black hole candidate. There are also rare Be–white dwarf systems. Only eight of these special systems are known to us today.

Some of these stars act like tiny versions of giant objects. We call these microquasars. They are like the smaller cousins of quasars.

Quasars have supermassive black holes that are millions of solar masses. Microquasars have compact objects that are only a few solar masses. These small versions can show radio jets. These jets shoot out from the center at very high speeds. They can even show apparent superluminal motion. Studying them helps us understand how big quasars work.

An X-ray binary is a special type of binary star system. In these systems, two stars orbit each other. One of these stars is a compact object. This compact object can be a white dwarf, a neutron star, or a black hole. The second star is called the donor star.

These systems are very important to astronomers. They are highly luminous in X-rays. This makes them some of the brightest objects in the X-ray sky. However, they are often relatively faint in visible light. This is because they emit almost all of their radiation as X-rays.

The X-rays are produced by a specific process of moving matter. Matter falls from the donor star toward the compact accretor. As this matter falls, it releases gravitational potential energy. This energy is converted into X-ray radiation. The process is incredibly efficient. The falling matter can release up to 30 percent of its rest mass as X-rays. For comparison, hydrogen fusion only releases about 0.7 percent of rest mass. In many systems, the infalling matter forms an accretion disk. This disk is the brightest part of the entire system. The mass-transfer rate depends on the orbital separation and the mass ratio. It also depends on the evolutionary status of the donor star.

Astronomers classify these systems into several subclasses based on the donor star. The classification by mass refers to the visible donor star, not the compact object. Low-mass X-ray binaries (LMXBs) feature a donor that is less massive than the compact object. These donors might be main sequence stars, red giants, or white dwarfs. LMXBs can show variability as X-ray bursters. These are thermonuclear explosions caused by the accretion of hydrogen and helium.

Intermediate-mass X-ray binaries (IMXBs) feature a star of medium mass. These systems may eventually evolve into LMXB systems.

High-mass X-ray binaries (HMXBs) contain a very massive donor star. These stars are often blue supergiants, O or B stars, or Wolf–Rayet stars. In these systems, the massive star dominates the visible light. However, the compact object remains the dominant source of X-rays. The compact object captures a fraction of the massive star's stellar wind. This process can be unstable and creates short-lived mass transfer. HMXBs often show variability as X-ray pulsars. This happens when magnetic fields funnel matter into the poles of the compact star.

One famous example is Cygnus X-1, the first identified black hole candidate.

There are even more specialized types of these systems. Be/X-ray binaries (BeXRBs) consist of a Be star and a neutron star. The neutron star usually follows a wide, highly elliptical orbit. When the neutron star passes through the Be star's disk, it creates a bright X-ray flare. Another rare type is the Be–white dwarf X-ray binary. Only eight of these systems are currently known. These form when mass transfer spins up the accretor to become a Be star.

Some HMXBs can eventually become double neutron star binaries if they are not interrupted by a supernova.

Microquasars are a fascinating subclass of X-ray binaries. They are often called radio-jet X-ray binaries. They are the smaller cousins of quasars. A microquasar has an accretion disk and often shows radio jets. These jets are relativistic, meaning they move at very high speeds. They can even show apparent superluminal motion.

While a quasar has a supermassive black hole, a microquasar has a compact object of only a few solar masses. Because they are smaller, they change much faster. A microquasar can experience changes in one day that would take a quasar centuries to show.

Studying these systems provides a bridge to understanding larger cosmic structures. Microquasars are very important for the study of relativistic jets. They allow scientists to see how matter behaves near a compact object on a faster timescale. We can see specific examples like SS 433, which shows atomic emission lines in its jets. Another is GRS 1915+105, which has a very high jet velocity. By looking at these small-scale versions, we learn about the physics of the most massive objects in the universe.