Scientists gain glimpse of bizarre matter in neutron star

NASA RELEASE


Don Savage
September 8, 2004
Headquarters, Washington, D.C.
(Phone: 202/358-1727)


Nancy Neal
Goddard Space Flight Center, Greenbelt, MD
(Phone: 301/286-0039)



Release 04-49

SCIENTISTS GAIN GLIMPSE OF BIZARRE MATTER IN A NEUTRON STAR

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'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/20...8nsmatter.html



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Jacques :-)

www.spacepatches.info