Accreting X-ray binaries

Most stars in the Universe are members of double (or multipe) star systems
orbiting one another in regular orbits.  Such double stars are called
binaries.  When one of the two stars is a massive star that explodes in a
supernova, it is possible for the tremendous explosion to tear apart the
binary.  In some cases the two stars stay together. Then the result is a
binary made up of one normal star and one collapsed star (either neutron
star or black hole).  Another way in which such a binary can form is when a
normal star and an already collapsed star accidentally meet, and capture
each other by their gravitational attraction.  In either case, when the
collapsed star orbits close enough to the ordinary one, gas from the
surface of the normal star will be attracted towards the collapsed one and
fall onto the surface of the neutron star or into the black hole.  This is
a violent process in which the gas first swirls around the neutron star or
black hole for a while, at speeds of up to more than a hundred thousand
kilometers per second, in tighter and tighter orbits.

In the process this flow of gas forms a flat disk of very hot plasma
emitting copious amounts of radiation of all wavelengths, dominated by
X-rays.  For this reason such binaries are called X-ray binaries.  The
neutron stars at the centers of such disks also become very hot, and the
swirling gas makes them spin faster and faster, producing X-ray pulses
spaced only miiliseconds apart. The black holes, of course, remain black
but the radiation from the plasma orbiting them carries the signature of
these enigmatic objects and their effects on the surrounding spacetime.
This radiation can be observed with astronomical techniques and used to
learn about the properties of neutron stars and black holes, and their
surroundings.  API astronomers use X-ray satellites to observe the
radiation from the hot plasma, and optical telescopes to look at the normal
stars.

neutron_star_accretion_rotation

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