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's Goddard Space Flight Center in Greenbelt, Md., and his colleague, Adam Villarreal, a graduate student at the University of Arizona, present these results today during a Web-based press conference in New Orleans at the meeting of the High Energy Astrophysics Division of the American Astronomical Society.

They said their best estimate of the radius of a neutron star is 7 miles (11.5 kilometers), plus or minus a stroll around the French Quarter.  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."   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 in today's announcement 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 neutron star spin rate 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 spin rate of 45 times per second.

Strohmayer and Villarreal next capitalized on EXO 0748-676 observations with the European Space Agency's XMM-Newton satellite from 2002, led by Dr. Jean Cottam of NASA Goddard.  These scientists had measured a mass-radius ratio, albeit without knowledge of a mass and radius.  This was based on the strength of gravity tugging at light particles.  The team also measured the speed of the gas using the Doppler-shift technique, just like a state trooper monitoring traffic. 

Using the new spin measurement and the Doppler shift technique, Strohmayer and Villarreal determined that the neutron star speed correlated to a radius between 9.5 and 15 kilometers, with the best estimate at 11.5 kilometers. Knowing the mass-radius ratio and now the radius, they calculated the star's mass 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 Villareal.  "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."

The proposed Constellation X-ray Observatory mission would have the ability to make such measurements, but with much greater precision, for a number of neutron star systems.  Images and more information about this result is available at:

http://www.gsfc.nasa.gov/topstory/2004/0908nsmatter.html