Scientists Gain Glimpse of Bizarre Matter in a Neutron Star
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issued by NASA Headquarters
September 8, 2004, 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.
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 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.Ê
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.Ê
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 spin rate of the neutron star
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
neutron star spin rate of 45 times per second. This is a leisurely pace for neutron stars,
which are often seen spinning over 300 times per second.
The scientists 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. Cottam's
team had detected spectral lines emitted by hot gas, similar in look to the lines 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.Ê
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, with the best estimate at 11.5 kilometers. The relationship among burst
frequency, Doppler shifting and radius is that the velocity of gas swirling around the
star's surface depends on the star's radius and its spin rate. In essence, a faster spin
corresponds to a wider spectral line (a technique similar to how a state trooper can
detect speeding cars).Ê
Cottam team's gravitational redshift measurement offered the first measure of a
mass-radius ratio, albeit without knowledge of a mass and radius. This is because the
degree of redshifting (strength of gravity) depends on the mass and radius of the neutron
star. Some scientists had questioned this measurement, for the spectral lines detected
seemed too narrow. The new results strengthen the gravitational redshift interpretation
of the Cottam team's spectral lines (and thus the mass-radius ratio) because a slower-
spinning star can easily produce such relatively narrow lines.
So, ever more confident of the mass-radius ratio and now knowing the radius, the scientists
could calculate the neutron star's mass. The value was 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."Ê
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.
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