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EXOTIC
INNARDS OF A NEUTRON STAR REVEALED IN A SERIES OF EXPLOSIONS
Amid
the fury of 28 thermonuclear blasts on a neutron star's surface,
scientists using the European Space Agency's (ESA) XMM-Newton X-ray
satellite have obtained a key measurement revealing the nature of matter
inside these enigmatic objects.
The
result, capturing for the first time the ratio between such an ultra-dense
star's mass and radius in an extreme gravity environment, is featured in
the November 7 issue of Nature. Dr. Jean Cottam of NASA's Goddard Space
Flight Center in Greenbelt, Md., leads this international effort.
The
neutron star -- the core remains of a star once bigger than the Sun yet
now small enough to fit within the Washington Beltway -- contains densely
packed matter under forces that perhaps existed at the moment of the Big
Bang but which cannot be duplicated on Earth. The contents offer a crucial
test for theories describing the fundamental nature of matter and energy.
Cottam
and her team probed the neutron star's interior by measuring for the first
time how light passing through the star's half-inch atmosphere is warped
by extreme gravity, a phenomenon called the gravitational redshift. The
extent of the gravitational redshift, as predicted by Einstein, depends
directly on the neutron star's mass and radius. The mass-to-radius ratio,
in turn, determines the density and nature of the star's internal matter,
called the equation of state.
"It
is only during these bursts that the region is suddenly flooded with light
and we were able to detect within that light the imprint, or signature, of
material under extreme gravitational forces," said Cottam.
The
neutron star 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
in the Milky Way galaxy, visible in southern skies with a large backyard
telescope.
Scientists
estimate that neutron stars contain the mass of about 1.4 Suns compacted
into about a 10-mile-wide sphere (16 kilometers). At such density, all the
space is squeezed out of the atoms inside the neutron star, and protons
and electrons squeeze into neutrons, leaving a neutron superfluid, a
liquid that flows without friction.
By
understanding the precise ratio of mass to radius, and thus pressure to
density, scientists can determine the nature of this superfluid and
speculate on the presence of exotic matter and forces within -- the type
of phenomena that particle physicists search for in earthbound particle
accelerators.
Today's
announcement states that EXO 0748-676's mass-to-radius ratio is 0.152
solar masses per kilometer, based on a gravitational redshift measurement
of 0.35. This provides the first observational evidence that neutron stars
are indeed made of tightly packed neutrons, as predicted by theory
estimating mass-radius, density-pressure ratios.
"Unlike
the Sun, with an interior well understood, neutron stars have been like a
black box," said co-author Dr. Frits Paerels of Columbia University
in New York. "We have bored our first small hole into a neutron star.
Now theorists will have a go at the little sample we have offered
them," he said.
More
important, said co-author Dr. Mariano Mendez of SRON, the National
Institute for Space Research in the Netherlands, "We have now
established a means to probe the bizarre interior of a 10-mile-wide chunk
of neutrons thousands of light-years away -- based on gravitational
redshift. With the fantastic light-collecting potential of XMM-Newton, we
can measure the mass-to-radius ratios of other neutron stars, perhaps
uncovering a quark star."
In a
quark star, which is denser than a neutron star and has a different
mass-to-radius ratio, neutrons are squeezed so tightly they liberate the
subatomic quark particles and gluons that are the building blocks of
atomic matter.
To
obtain its measurement, the team needed the fantastic radiance provided by
thermonuclear bursts, which illuminate matter very close to the neutron
star surface where gravity is strongest. The team spotted the 28 bursts
during a series of XMM-Newton observations of the neutron star totaling 93
hours. There are dozens of known binary systems with neutron stars, like
EXO 0748-676, where such bursting is seen several times a day, the result
of gas pouring onto the neutron star from its companion star.
ESA's
XMM-Newton was launched in December 1999. NASA helped fund mission
development and supports guest observatory time. Goddard Space Flight
Center hosts the U.S. guest visitor-support center. Jean Cottam joins
Goddard through a grant from the National Research Council.
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