Binary stars that pulse and flash
Illustration of matter flowing into the
magnetic field of a neutron star
If the X-ray binary contains a neutron star, it may be seen to pulse in
X-rays or give off bursts of X-rays. In the artist's impression seen here, the
material is funneled by the strong magnetic field of the neutron star onto
its magnetic poles. The magnetic fields on neutron stars can be as high as
1,000 billion times that of the Earth. These are the strongest magnetic
fields known anywhere in the universe, and X-ray astronomy is the only way to
study them. Such strong magnetic fields cause the X-rays to appear 'beamed',
so that the X-ray pulsar appears as a "flashing lighthouse". In
addition, the material falling in from the companion will hit a hard surface
and this will shine in X-rays as an intense 'hot spot'. When X-ray
observatories observe these systems, the hot spot near the magnetic pole will
flash as the neutron star rotates. This is seen in the figure below.
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Light curve of the binary system 4U1705-44 illustrating the thermonuclear
X-ray flashes from the neutron star.
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Not all neutron star systems, however, have intense magnetic fields. It
appears that in many of the low mass X-ray binary systems, the field is much
weaker. In these cases, the flow of material onto the neutron star is more
erratic, with the pulsations unstable. Many of these low magnetic field
systems show what are called "quasi-periodic oscillations" (QPO).
These oscillations seem to originate from the interaction of the neutron star
with the surrounding
accretion
disk, or from instabilities in the captured material caused by the
overwhelming intensity of the X-ray emission. Study of QPO in low mass
binaries provides astronomers new insight into how the neutron star interacts
with its surroundings.
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