Among the myriad stars of our Galaxy, a handful are observed regularly emitting pulses of X-rays. The pulsing X-rays are detected with instruments on spacecraft orbiting above the Earth’s atmosphere since the atmosphere absorbs X-rays and prevent any from reaching the Earth’s surface. These pulsing X-ray stars or X-ray pulsars actually consist of a pair of stars orbiting around each other. One member of the pair is a small, solid star commonly known as a neutron star while the other is a normal gaseous star like our Sun, but usually many times more massive. The neutron star is only a few dozen kilometers in size but has a mass roughly equal to the Sun.
The neutron star is the collapsed core of a giant star that ran out of nuclear fuel at an earlier time. Normal stars, like our Sun, are composed of hot gas that, like any unconfined gas will try and expand to fill up space. On the other hand, the gravity of the star tries to collapse the gas under its own weight. The balance between the forces of expansion and collapse produce an ordinary stable star. However, the balance can’t last forever since stars are cooling down by emitting radiation into space; in other words they shine! Their gas remains hot by “burning” hydrogen gas in their cores by the process of nuclear fusion. Eventually all stars use up their fuel supply, cool off and collapse. The collapsed remnant of stars within a certain mass range will be neutron stars while less massive stars become white dwarfs and more massive stars collapse into black holes.
The material in a relatively small body like the Earth is made of atoms. The atoms contain lightweight electrons orbiting around a small, heavy nucleus of protons and neutrons. But the pressures in a neutron star are so great that the atoms themselves collapse and the electrons are absorbed by the nuclei. The material in a neutron star is composed of atomic nuclei compressed tightly together and contains mostly neutrons; hence the name neutron star. The weight of the star attempts to further compress the neutrons together. But the neutrons respond to the increasing confinement not by squeezing closer together, but instead by moving faster, as prescribed by a fundamental quantum mechanical property known as the Heisenberg uncertainty principle. The neutrons thus act much like the particles in a hot gas and provide a counter pressure (technically known as “degeneracy pressure”) that resists further gravitational collapse. Since this pressure is only the result of confinement of neutrons and not due to heating, the neutron star can exist as a permanent object like the Earth.
Many neutron stars have magnetic fields that are more than one trillion times stronger than the familiar field on the surface of the Earth that moves compass needles. In this respect, these neutron stars are like giant rotating bar magnets. When in orbit with a normal stellar companion, gas from the companion can be pulled onto the neutron star. The hot gas falling onto the neutron star is channeled by the magnetic field onto the North and South magnetic poles. Thus two “hot spots” on the neutron star surface are formed akin to the two auroral zones on the Earth but far hotter. In fact, at these hotspots the infalling gas can reach half the speed of light before it impacts the surface. So much energy is released by the infalling gas that the hotspots, which are only a few hundred square meters in size, can be up to ten thousand times brighter than the Sun! Temperatures of millions of degrees are produced so the hotspots emit mostly X-rays. As the neutron star rotates, we observe pulses of X-rays.
The gas that supplies the X-ray pulsar can reach the neutron star by a variety of ways that depend on the size and shape of the neutron star’s orbital path and the nature of the companion star. Some companion stars of X-ray pulsars are very massive young stars that emit a stellar wind from their surface. The neutron star is immersed in the wind and continuously captures gas that flows nearby. In other systems, the neutron star orbits so closely to its companion that its strong gravitational force can pull material from the companion’s atmosphere into an orbit around itself. This material forms a gaseous disk in which material spirals inwards to ultimately fall onto the neutron star. For still other types of X-ray pulsars, the companion star is rotating very rapidly and apparently shedding a disk of gas around its equator. The orbits of the neutron star with these companions are usually large and elliptical in shape. When the neutron star passes nearby or through the disk it will capture material and temporarily become an X-ray pulsar. The circumstellar disk expands and contracts for unknown reasons, so these temporary or transient X-ray pulsars are observed only intermittently often with months to years between episodes of X-ray pulsation.
