Amid the fury of 28 thermonuclear blasts on a neutron
\nstar’s surface, scientists using the European Space Agency’s
\n(ESA) XMM-Newton X-ray satellite have obtained a key
\nmeasurement revealing the nature of matter inside these
\nenigmatic objects.
\n
\n
\nThe result, capturing for the first time the ratio between
\nsuch an ultra-dense star’s mass and radius in an extreme
\ngravity environment, is featured in the November 7 issue of
\nNature. Dr. Jean Cottam of NASA’s Goddard Space Flight Center
\nin Greenbelt, Md., leads this international effort.<\/p>\n
\nThe neutron star — the core remains of a star once bigger
\nthan the Sun yet now small enough to fit within the
\nWashington Beltway — contains densely packed matter under
\nforces that perhaps existed at the moment of the Big Bang but
\nwhich cannot be duplicated on Earth. The contents offer a
\ncrucial test for theories describing the fundamental nature
\nof matter and energy. <\/p>\n
\nCottam and her team probed the neutron star’s interior by
\nmeasuring for the first time how light passing through the
\nstar’s half-inch atmosphere is warped by extreme gravity, a
\nphenomenon called the gravitational redshift. The extent of
\nthe gravitational redshift, as predicted by Einstein, depends
\ndirectly on the neutron star’s mass and radius. The mass-to-
\nradius ratio, in turn, determines the density and nature of
\nthe star’s internal matter, called the equation of state. <\/p>\n
“It is only during these bursts that the region is suddenly
\nflooded with light and we were able to detect within that
\nlight the imprint, or signature, of material under extreme
\ngravitational forces,” said Cottam. <\/p>\n
The neutron star is part of a binary star system named EXO
\n0748-676, located in the constellation Volans, or Flying
\nFish, about 30,000 light-years away in the Milky Way galaxy,
\nvisible in southern skies with a large backyard telescope.<\/p>\n
\nScientists estimate that neutron stars contain the mass of
\nabout 1.4 Suns compacted into about a 10-mile-wide sphere (16
\nkilometers). At such density, all the space is squeezed out
\nof the atoms inside the neutron star, and protons and
\nelectrons squeeze into neutrons, leaving a neutron
\nsuperfluid, a liquid that flows without friction. <\/p>\n
By understanding the precise ratio of mass to radius, and
\nthus pressure to density, scientists can determine the nature
\nof this superfluid and speculate on the presence of exotic
\nmatter and forces within — the type of phenomena that
\nparticle physicists search for in earthbound particle
\naccelerators. <\/p>\n
Today’s announcement states that EXO 0748-676’s mass-to-
\nradius ratio is 0.152 solar masses per kilometer, based on a
\ngravitational redshift measurement of 0.35. This provides the
\nfirst observational evidence that neutron stars are indeed
\nmade of tightly packed neutrons, as predicted by theory
\nestimating mass-radius, density-pressure ratios. <\/p>\n
“Unlike the Sun, with an interior well understood, neutron
\nstars have been like a black box,” said co-author Dr. Frits
\nPaerels of Columbia University in New York. “We have bored
\nour first small hole into a neutron star. Now theorists will
\nhave a go at the little sample we have offered them,” he
\nsaid. <\/p>\n
More important, said co-author Dr. Mariano Mendez of SRON,
\nthe National Institute for Space Research in the Netherlands,
\n“We have now established a means to probe the bizarre
\ninterior of a 10-mile-wide chunk of neutrons thousands of
\nlight-years away — based on gravitational redshift. With the
\nfantastic light-collecting potential of XMM-Newton, we can
\nmeasure the mass-to-radius ratios of other neutron stars,
\nperhaps uncovering a quark star.” <\/p>\n
In a quark star, which is denser than a neutron star and has
\na different mass-to-radius ratio, neutrons are squeezed so
\ntightly they liberate the subatomic quark particles and
\ngluons that are the building blocks of atomic matter. <\/p>\n
To obtain its measurement, the team needed the fantastic
\nradiance provided by thermonuclear bursts, which illuminate
\nmatter very close to the neutron star surface where gravity
\nis strongest. The team spotted the 28 bursts during a series
\nof XMM-Newton observations of the neutron star totaling 93
\nhours. There are dozens of known binary systems with neutron
\nstars, like EXO 0748-676, where such bursting is seen several
\ntimes a day, the result of gas pouring onto the neutron star
\nfrom its companion star.<\/p>\n
ESA’s XMM-Newton was launched in December 1999. NASA helped
\nfund mission development and supports guest observatory time.
\nGoddard Space Flight Center hosts the U.S. guest visitor-
\nsupport center. Jean Cottam joins Goddard through a grant
\nfrom the National Research Council. <\/p>\n