Above: a binary star system in which a small, compact and very dense neutron star sucks material from its
companion star, forming an accretion disc and gaseous torus (doughnut-ring shape) around the neutron
star, as material slowly spirals onto it. Note the hot spot where the jet of material leaving the donor star
impacts on the edge of the accretion disc / torus.
Most stars exist as binary (or double) stars in which two stars orbit one another (around their common centre
of gravity). Often one of the two stars will be much larger (more massive) than the other one, and since
larger stars have more weight above their cores, their cores are hotter and at higher pressure. These
conditions favour nuclear reactions, and nuclear fusion occurs faster in larger stars. Despite having more
fuel to burn, larger stars get through their fuel much more quickly than young stars. In the case of a binary
system, the larger star is most likely to reach the red giant stage long before its smaller companion.
Often this will result in the formation of a cataclysmic variable, in which the old star becomes a white dwarf.
However, if the star is left with too much material to exist as a white dwarf, then it will collapse into a neutron
star or even a black hole.
Neutron stars, and especially black holes are extremely dense and have immensely strong gravitational
fields. If the companion star is too close, then it will overflow its Roche lobe (the boundary at which material is
equally attracted to either star by their gravitational fields) and this overflowing material will be more strongly
attracted, by gravity, to the compact companion, rather than to the donor star. This material will stream
toward the neutron star or black hole, orbiting as it does so and forming an accretion disc of material which is
slowly falling in to the recipient star, which feeds off its companion like a parasite.
As material spirals in, inside the disc, it heats up as gravitational potential energy is converted into heat. In
the case of a neutron star or black hole, the gravitational field is so strong, that this material gets very hot
and when it impacts the neutron star it will emit very energetic X-rays. X-rays are essentially very energetic
light rays - blue light is more energetic than red light, invisible ultraviolet light is even more energetic, and
X-rays are more energetic still. These intense X-rays become stronger during episodes of rapid mass
transfer and diminish at quieter intervals.
The evidence for a standard accretion disc, like that in cataclysmic variables (CVs), in these systems is not
clearcut. It appears that instead of the thin disc we saw for the CV, these systems have thicker structures.
Simple models have a thick disc, or a torus, like that shown below:
However, such models are generally considered physically unrealistic and unlikely. More likely is an accretion
disc surrounded by a torus of clouds, as shown in the main image at top, or a thin disc which is warped and
so has a bulge in it. Another model imagines the stream of incoming material skimming over the top of a thin
accretion disc and then spreading out to form an elevated ring part-way along the disc.
Additional complications come from the consideration of magnetic fields. Some neutron stars have immense
magnetic fields (up to 10^11Tesla or 10^15 gauss, compared to the Earth's magnetic field at 0.3 to 0.6
gauss) and are called magnetars. As material spirals in to the neutron star from the accretion disc, it
reaches a point where the magnetic field becomes so intense, that it disrupts the disc and material is forced
to flow along magnetic field lines toward the neutron star, and so probably impacts near the magnetic poles
of the star. This is shown in the main image, as the disc gives way to a largely empty region immediately
around the neutron star in the middle of the disc and from their flows in one or more streams to the neutron
star. This could well be accompanied by jets (which may be episodic) of hot material blasting off from the
poles of the neutron star.
The Earth acts rather like a giant bar magnet - it has a magnetic North pole near to the geographic North
Pole and a South magnetic pole near to the geographic South Pole. It is this magnetic field which causes a
magnetised compass needle to point to magnetic North. The magnetic field of the Earth varies, on the Earth's
surface, from about 0.3 to 0.6 G (G = gauss). This is a strong magnetic field, but the magnetic field of a
magnetar may be more than one million billion times as strong (at 1 000 000 000 000 000 G)! Clearly a
severe safety hazard to passing spaceships! Some white dwarfs may have magnetic fields up to several
billion times that of the Earth, which is also immense, but tiny compared to that of a magnetar. The most
powerful superconducting magnets developed by humans has a strength of about 130 000 G and the
strongest pulsed magnets at 720 000 G - very weak in comparison!