Chandra Finds Link to Origin of Millisecond Pulsars

Chandra Finds Long-Sought Link to Origin of Millisecond Pulsars


The peculiar cosmic object known as 47 Tuc W (denoted by arrow in the X-ray
image) is a double star system consisting of a normal star and a neutron
star that makes a complete rotation every 2.35 milliseconds. Blink your eye
and a superdense star the size of Manhattan Island will have rotated 25 or
more times!

New Chandra observations give the best information yet on why such neutron
stars, called millisecond pulsars, are rotating so fast. The key, as in real
estate, is location, location, location - in this case the crowded confines
of the globular star cluster 47 Tucanae, where stars are less than a tenth
of a light year apart. Almost two dozen millisecond pulsars are located
there. This large sample is a bonanza for astronomers seeking to test
theories for the origin of millisecond pulsars, and increases the chances
that they will find a critical transitional object such 47 Tuc W.



47 Tuc W stands out from the crowd because it produces more high-energy
X-rays than the others. This anomaly points to a different origin of the
X-rays, namely a shock wave due to a collision between matter flowing from a
companion star and particles racing away from the pulsar at near the speed
of light. Regular variations in the optical and X-ray light corresponding to
the 3.2-hour orbital period of the stars support this interpretation.

A team of astronomers from the Harvard-Smithsonian Center for Astrophysics
in Cambridge, MA pointed out that the X-ray signature and variability of the
light from 47 Tuc W are nearly identical to those observed from an X-ray
binary source known as J1808. They suggest that these similarities between a
known millisecond pulsar and a known X-ray binary provide the long-sought
link between these types of objects.

In theory, the first step toward producing a millisecond pulsar is the
formation of a neutron star when a massive star goes supernova. If the
neutron star is in a globular cluster, it will perform an erratic dance
around the center of the cluster, picking up a companion star which it may
later swap for another.

As on a crowded dance floor, the congestion in a globular cluster can cause
the neutron star to move closer to its companion, or to swap partners to
form an even tighter pair. When the pairing becomes close enough, the
neutron star begins to pull matter away from its partner. As matter falls
onto the neutron star, it gives off X-rays. An X-ray binary system has been
formed, and the neutron star has made the crucial second step toward
becoming a millisecond pulsar.



The matter falling onto the neutron star slowly spins it up, in the same way
that a child's carousel can be spun up by pushing it every time it comes
around. After 10 to 100 million years of pushing, the neutron star is
rotating once every few milliseconds. Finally, due to the rapid rotation of
the neutron star, or the evolution of the companion, the infall of matter
stops, the X-ray emission declines, and the neutron star emerges as a
radio-emitting millisecond pulsar.

It is likely that the companion star in 47 Tuc W - a normal star with a mass
greater than about an eighth that of the Sun - is a new partner, rather than
the companion that spun up the pulsar. The new partner, acquired fairly
recently in an exchange that ejected the previous companion, is trying to
dump on the already spun-up pulsar, creating the observed shock wave. In
contrast, the X-ray binary J1808 is not in a globular cluster, and is very
likely making do with its original companion, which has been depleted to a
brown dwarf size with a mass less than 5% that of the Sun.

Most astronomers accept the binary spin-up scenario for creating millisecond
pulsars because they have observed neutron stars speeding up in X-ray binary
systems, and almost all radio millisecond pulsars are observed to be in
binary systems. Until now, definitive proof has been lacking, because very
little is known about transitional objects between the second and final
steps.

That is why 47 Tuc W is hot. It links a millisecond pulsar with many of the
properties of an X-ray binary, to J1808, an X-ray binary that behaves in
many ways like a millisecond pulsar, thus providing a strong chain of
evidence to support the theory.


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Jacques :-)

www.spacepatches.info