With NASA’s Chandra X-ray Observatory, astronomers have detected features
that may be the first direct evidence of the effect of gravity on radiation
from a neutron star. This finding, if confirmed, could enable scientists
to measure the gravitational field of neutron stars and determine whether
they contain exotic forms of matter not seen on Earth.

A team led by George Pavlov of Penn State University in University
Park observed 1E 1207.4-5209, a neutron star in the center of a supernova
remnant about 7,000 light years from Earth. The results were presented
on June 6, 2002, at the American Astronomical Society in Albuquerque,

Pavlov’s group found two dips, or absorption features, in the spectrum
of X-rays from the star. If these dips are due to the absorption of
X-rays near the star by helium ions in a strong magnetic field, they
indicate that the gravitational field reduces the energies of X-rays
escaping from near the surface of a neutron star.

"This interpretation is consistent with the data," said Pavlov,
"but the features may be a blend of many other features. More precise
measurements, preferably with Chandra’s grating spectrometer, are needed."

 "These absorption features may be the first evidence of the effect
of gravity on radiation near the surface of an isolated neutron star,"
said Pavlov. "This is particularly important because it would allow
us to set limits on the type of matter that comprises this star."

Neutron stars are formed when a massive star runs out of fuel and its
core collapses. A supernova explosion occurs and the collapsed core
is compressed to a hot object about 12 miles in diameter, with a thin
atmosphere of hydrogen and possibly heavier ions in a gravitational
field 100 billion times as strong as Earth’s.

These objects, which have a density of more than 1 billion tons per
teaspoonful, are called neutron stars because they have been thought
to be composed mostly of neutrons. Although neutron stars have been
studied extensively for more than three decades, their exact nature
is still unknown.

"We are not even sure that neutron stars are composed of neutrons,"
said Divas Sanwal, also of Penn State, and lead author on a paper describing
the team’s results. "They could be largely composed of subatomic
particles called pions or kaons, or even free quarks."

One key to narrow the range of possibilities is to measure the strength
of gravity on the surface of a neutron star by observing its effect
on X-rays from very near the surface of the star. According to Einstein’s
theory of General Relativity, attraction of photons by a star’s gravitational
field results in a lower energy of the photon (longer wavelength of
radiation) when detected by a distant observer. The measurement of this
gravitational redshift relates the mass to the radius of the star, and
it will test the theories for the various possible forms of dense matter.

The team, which also includes Slava Zavlin of Max Plank Institute for
Extraterrestrial Physics, Germany, and Marcus Teter of Penn State, considered
several possible explanations for the absorption features observed from
1E 1207. The strength and X-ray energy of the features make it improbable
that they are due to intervening interstellar material or absorption
due to electrons or ions circling in a strong magnetic field. The most
likely hypothesis, they conclude, is that the features are due to absorption
by helium ions in a magnetic field about a hundred trillion times more
intense than the Earth’s magnetic field. In this case, the gravitational
redshift reduces the energy of the X-rays by 17 percent.

Pavlov and his colleagues observed 1E 1027 with Chandra’s Advanced
CCD Imaging Spectrometer on January 6, 2000, and again on January 5,
2002, each time for approximately 30,000 seconds.

The ACIS instrument was built for NASA by Penn State and the Massachusetts
Institute of Technology, Cambridge, Mass. under the leadership of Gordon
Garmire of Penn State. NASA’s Marshall Space Flight Center, Huntsville,
Ala., manages the Chandra program for the Office of Space Science, Washington,
D.C. TRW, Inc., Redondo Beach, Calif., is the prime contractor for the
spacecraft. The Smithsonian’s Chandra X-ray Center controls science
and flight operations from Cambridge.

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