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 labeled 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.
Credit NASA/CXC/Michigan State/A.Steiner et al
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.
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 labeled 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.
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 Center in Huntsville, Ala., 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, Mass.
Fast Facts for 47 Tucanae:
Release Date: March 6, 2013
Scale: Image is 2.3 arcmin across (about 10 light years)
Category: Neutron Stars/X-ray Binaries
Coordinates: (J2000) RA 00h 24m 42.0s | Dec -72° 00' 00"
Constellation: Tucana
Observation Date: 13 pointings between March 16, 2000 and Oct 11, 2002
Observation Time: 100 (4 days, 4 hours).
Obs. ID: 78, 953-956, 2735-2738, 3384-3387
Instrument: ACIS
References: Steiner, A. et al 2013, ApJ 765, L5; arXiv:1205.6871
Color Code: X-ray (Red, Green, Blue)
