ByANDY FLEMING
Neutron stars, the ultra-dense cores left behind
after massive stars collapse, contain the densest matter known in the Universe
outside of a black hole. New results from Chandra and other X-ray telescopes have
provided one of the most reliable determinations yet of the relation between
the radius of a neutron star and its mass. These results constrain how nuclear
matter - protons and neutrons, and their constituent quarks - interact under
the extreme conditions found in neutron stars.
Three telescopes - Chandra, ESA's XMM-Newton,
and NASA's Rossi X-ray Timing Explorer (RXTE) - were used to observe 8 neutron
stars, including one in 47 Tucanae, a globular cluster located about 15,000
light years away in the outskirts of the Milky Way. The image shown here was
constructed from a long Chandra observation of 47 Tucanae. Lower-energy X-rays
are red, X-rays with intermediate energies are green, and the highest-energy
X-rays are shown in blue.
In the image, the double, or binary, star system
labelled as X7 contains a neutron star slowly pulling gas away from a companion
star with a mass much lower than the Sun. In 2006, researchers used
observations of the amount of X-rays from X7 at different energies together
with theoretical models to determine a relationship between the mass and the
radius of the neutron star. A similar procedure was used for Chandra
observations of a neutron star in another globular cluster, NGC 6397, and for
two other neutron stars in clusters observed by ESA's XMM-Newton.
NGC6397
Four other neutron stars were observed with RXTE
to undergo bursts of X-rays that cause the atmosphere of the neutron star to
expand. By following the cooling of the star, its surface area can be
calculated. Then, by folding in independent estimates of the distance to the
neutron star, scientists were able to gather more information on the
relationships between the masses and radii of these neutron stars.
Because the mass and radius of a neutron star is
directly related to interactions between the particles in the interior of the
star, the latest results give scientists new information about the inner
workings of neutron stars.
The researchers used a wide range of different
models for the structure of these collapsed objects and determined that the
radius of a neutron star with a mass that is 1.4 times the mass of the Sun is
between 10.4 and 12.9 km (6.5 to 8.0 miles). They also estimated the density at
the center of a neutron star was about 8 times that of nuclear matter found in
Earth-like conditions. This translates into a pressure that is over ten
trillion trillion times the pressure required for diamonds to form inside the
Earth.
The results apply whether the entire set of
bursting sources, or the most extreme of the other sources, are removed from
the sample. Previous studies have used smaller samples of neutron stars or have
not accounted for as many uncertainties in using the models.
The new values for the neutron star's structure
should hold true even if matter composed of free quarks exists in the core of
the star. Quarks are fundamental particles that combine to form protons and
neutrons and are not usually found in isolation. It has been postulated that
free quarks may exist inside the centers of neutron stars, but no firm evidence
for this has ever been found.
The researchers also made an estimate of the
distances between neutrons and protons in atomic nuclei here on earth. A larger
neutron star radius naturally implies that, on average, neutrons and protons in
a heavy nucleus are farther apart. Their estimate is being compared with values
from terrestrial experiments.
The neutron star observations also provided new
information about the so-called "symmetry energy" for nuclear matter,
which is the energy cost required to create a system with a different number of
protons than neutrons. The symmetry energy is important for neutron stars
because they contain almost ten times as many neutrons as protons. It is also
important for heavy atoms on Earth, like Uranium, because they often have more
neutrons than protons. The results show that the symmetry energy does not
change much with density.
These results will be published in a paper in
the March 1st, 2013 issue of The Astrophysical Journal Letters. The authors are
Andrew Steiner, from the Institute for Nuclear Theory at the University of
Washington, James Lattimer from Stony Brook University in New York and Edward
Brown from Michigan State University.
NASA's Marshall Space Flight Centre in
Huntsville, Alabama, manages the Chandra program for NASA's Science Mission Directorate
in Washington. The Smithsonian Astrophysical Observatory controls Chandra's
science and flight operations from Cambridge, Massachussetts.
Original Source: NASA
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