Scientists Gain Glimpse of Bizarre Matter in a Neutron Star

Neutron star EXO 0748-676 (blue sphere) is part of a binary star system, and its neighboring star (yellow-red sphere) supplies the fuel for the thermonuclear bursts. (Image Credit: NASA) QuickTime Movie (8.7 MB) Description

Scientists have obtained their best measurement yet of the size and contents of a neutron star, an ultra-dense object containing the strangest and rarest matter in the Universe.

This measurement may lead to a better understanding of nature's building blocks -- protons, neutrons and their constituent quarks -- as they are compressed inside the neutron star to a density trillions of times greater than on Earth.

Dr. Tod Strohmayer of NASA Goddard Space Flight Center in Greenbelt, Md., and his colleague, Adam Villarreal, a graduate student at the University of Arizona, presented these results at the meeting of the High Energy Astrophysics Division of the American Astronomical Society in early September, 2004.

They said their best estimate of the radius of a neutron star is about 11.5 kilomenters (7 miles). The mass appears to be 1.75 times that of the Sun, more massive than some theories predict. They made their measurements with NASA's Rossi X-ray Timing Explorer and archived X-ray data.

The long-sought mass-radius relation defines the neutron star's internal density and pressure relationship, the so-called equation of state. And this, in turn, determines what kind of matter can exist inside a neutron star. The contents offer a crucial test for theories describing the fundamental nature of matter and energy and the strength of nuclear interactions.

"We would really like to get our hands on the stuff at the center of a neutron star," said Strohmayer. "But since we can't do that, this is about the next best thing. A neutron star is a cosmic laboratory and provides the only opportunity to see the effects of matter compressed to such a degree."

Thermonuclear bursts arise as gas moving at close to the speed of light crashes onto the neutron star surface. (Image Credit: NASA)

A neutron star is the core remains of a star once bigger than the Sun. The interior contains matter under forces that perhaps existed at the moment of the Big Bang but which cannot be duplicated on Earth. The neutron star Strohmayer studied is part of a binary star system named EXO 0748-676, located in the constellation Volans, or Flying Fish, about 30,000 light-years away, visible in southern skies with a large backyard telescope.

In this system, gas from a "normal" companion star plunges onto the neutron star, attracted by gravity. This triggers thermonuclear explosions on the neutron star surface that illuminate the region. Such bursts often reveal the spin rate of the neutron star through a flickering in the X-ray light emitted, called a burst oscillation. The scientists detected a 45-hertz burst oscillation frequency, which corresponds to a neutron star spin rate of 45 times per second. This is a leisurely pace for neutron stars, which are often seen spinning over 300 times per second.

The scientists next capitalized on observations of EXO 0748-676 taken with the European Space Agency's XMM-Newton satellite in 2002, led by Dr. Jean Cottam of NASA Goddard. Cottam's team had detected spectral lines emitted by hot gas, similar in look to the lines of a cardiogram. These lines had two features. First, they were Doppler shifted. This means the energy detected was an average of the light spinning around the neutron star, moving away from us and then toward us. Second, the lines were gravitationally redshifted. This means that gravity pulled on the light as it tried to escape the region, stealing a bit of its energy.

Strohmayer and Villarreal determined that the 45-hertz frequency and the observed line widths from Doppler shifting are consistent with a neutron star radius between 9.5 and 15 kilometers, with the best estimate at 11.5 kilometers. The relationship among burst frequency, Doppler shifting and radius is that the velocity of gas swirling around the star's surface depends on the star's radius and its spin rate. In essence, a faster spin corresponds to a wider spectral line (a technique similar to how a state trooper can detect speeding cars).

Cottam team's gravitational redshift measurement offered the first measure of a mass-radius ratio, albeit without knowledge of a mass and radius. This is because the degree of redshifting (strength of gravity) depends on the mass and radius of the neutron star. Some scientists had questioned this measurement, for the spectral lines detected seemed too narrow. The new results strengthen the gravitational redshift interpretation of the Cottam team's spectral lines (and thus the mass-radius ratio) because a slower- spinning star can easily produce such relatively narrow lines.

The bursting neutron star with its companion star. (Image Credit: NASA)

So, ever more confident of the mass-radius ratio and now knowing the radius, the scientists could calculate the neutron star's mass. The value was between 1.5 and 2.3 solar masses, with the best estimate at 1.75 solar masses.

The result supports the theory that matter in the neutron star in EXO 0748-676 is packed so tightly that almost all protons and electrons are squeezed into neutrons, which swirl about as a superfluid, a liquid that flows without friction. Yet the matter isn't packed so tightly that quarks are liberated, a so-called quark star.

"Our results are really starting to put the squeeze on the neutron star equation of state," said Villarreal. "It looks like equations of state which predict either very large or very small stars are nearly excluded. Perhaps more exciting is that we now have an observational technique that should allow us to measure the mass-radius relations in other neutron stars."

A proposed NASA mission called the Constellation X-ray Observatory would have the ability to make such measurements, but with much greater precision, for a number of neutron star systems.