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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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