X-ray binary

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.