NEW ORLEANS — 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.

The 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.

Tod Strohmayer of NASA Goddard Space Flight Center in Greenbelt, Md., and
Adam Villarreal, a physics graduate student at the University of Arizona,
present their results today at the American Astronomical Society’s High
Energy Astrophysics Division meeting in New Orleans.

Strohmayer and Villarreal estimate that the neutron star is about 1.8 times
as massive as the sun — slightly more massive than expected — with a
radius of about 7 miles (11.5 kilometers). It is part of a binary star
system named EXO 0748-676, located about 30,000 light-years away in the
southern sky constellation Volans, or Flying Fish.

The scientists used NASA Rossi X-ray Timing Explorer data to measure how
fast the neutron star spins. Spin-rate was the unknown factor needed to
estimate the neutron star’s size and total mass. Their results agree with a
mass-to-radius ratio estimate made from European Space Agency (ESA) X-ray
satellite observations in 2002.

The long-sought mass-radius ratio defines the neutron star’s internal
density and pressure relationship, the so-called equation of state.

“Astrophysicists have been trying for decades to constrain the equation of
state of neutron star matter,” Villarreal said. “Our results hold great
promise for accomplishing this goal. It looks like equations of state which
predict either very large or very small stars are nearly excluded.”

Knowing a neutron star’s equation of state allows physicists to determine
what kind of matter can exist within that star. Scientists need to
understand such exotic matter to test 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 remnant 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.

In this system, gas from a “normal” companion star, attracted by gravity,
plunges onto the neutron star. 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
emission, called a burst oscillation.

Strohmayer and Villarreal 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 at more
than 600 times per second.

They next capitalized on EXO 0748-676 observations with ESA’s XMM-Newton
satellite, led by Jean Cottam of NASA Goddard in 2002. Cottam’s team
detected spectral lines emitted by hot gas, lines resembling those 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 towards 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. The
gravitational redshift measurement offered the first estimate of a
mass-radius ratio, because the degree of redshifting depends on the mass and
radius of the neutron star.

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 (between about 6 and 9 miles) with
the best estimate at 11.5 kilometers (about 7 miles). They used the radius
and the mass-radius ratio to calculate the neutron star’s mass between 1.5
and 2.3 solar masses, with the best estimate at around 1.8 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 together to become 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.

“Perhaps most exciting is that we now have an observational technique that
should allow us to measure the mass-radius relations in other neutron
stars,” Villarrael said.

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.