Dense Matter Of Neutron Stars Revealed Through X-Ray Observations

April Flowers for redOrbit.com – Your Universe Online

The densest matter in the Universe outside of black holes is contained in neutron stars, the ultra-dense cores left behind after massive stars collapse. One of the most reliable determinations yet of the relationship between the radius of a neutron star and its mass has been provided by new results from NASA’s Chandra, along with European Space Agency’s (ESA) XMM-Newton and NASA’s Rossi X-ray Timing Explorer (RXTE), which constrain how nuclear matter interact under the extreme conditions found in neutron stars.

The results of this study were published in a recent issue of The Astrophysical Journal Letters.

The three telescopes were used to observe eight neutron stars, including one located in 47 Tucanae. 47 Tucanae is a global cluster located about 15,000 light years away in the outskirts of the Milky Way. A long Chandra observation was used to construct an image of 47 Tucanae where the X-ray wavelengths are represented by different colors. Red represents the lowest-energy X-rays, intermediate level energies are green, and blue represents the highest-energy X-rays.

The binary star system labeled X7 in the image contains a neutron star slowly pulling gas away from its companion star, which has a mass much lower than the Sun. Researchers used observations of the amount of X-rays from X7 at different energies, along with theoretical models, to determine a relationship between the mass and the radius of the neutron star in 2006. Chandra observations of a neutron star in the globular cluster NGC 6397, and two neutron stars in clusters observed by ESA’s XMM-Newton, used similar procedures.

RXTE observed four additional neutron stars undergo bursts of X-rays that caused the atmosphere of each neutron star to expand. The surface area of the star can be calculated by following the cooling of the star after this expansion. Scientists then folded in independent estimates of the distance to the neutron star to gather more information on the relationships between the masses and the radii of these stars.

The latest results give scientists new information about the inner workings of 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 research team determined that the radius of a neutron star with a mass that is 1.4 times the mass of the Sun is between 6.5 to 8.0 miles using a wide range of different models for the structure of these collapsed objects. In addition, they estimated the density at the center of a neutron star to be about 8 times that of nuclear matter found in Earth-like conditions, translating into a pressure that is over ten trillion trillion times the pressure required for the formation of diamonds inside the Earth.

Whether the entire set of bursting sources, or the most extreme of the other sources, are removed from the sample, the results still apply. Prior studies have employed smaller samples of neutron stars or have not accounted for as many uncertainties in using the models.

Even if matter composed of free quarks exists in the core of the star, the new values for the star’s structure should hold true. Not usually found in isolation, quarks are the fundamental particles that combine to form protons and neutrons. Scientists theorize that free quarks may exist inside the centers of neutron stars, but there is no firm evidence yet.

The research team estimated the distances between neutrons and protons in atomic nuclei here on Earth as well. The implication of a larger neutron star radius is that, on average, neutrons and protons in a heavy nucleus are farther apart. The team’s estimate is being compared with values compiled from terrestrial experiments.

New information was also provided about the so-called “symmetry energy” for nuclear matter. This symmetry energy is the energy cost required to create a system with a different number of protons than neutrons. This energy is important for neutron stars as they contain nearly ten times as many neutrons as protons. Heavy atoms on Earth, such as uranium, often have more neutrons than protons, making symmetry energy important in understanding them as well. The results of the new study demonstrate that the symmetry energy does not change much with density.