Three Satellites Needed to Discover One Shy Star
An international team of scientists has uncovered a rare type of neutron
star so elusive that it took three satellites to identify it. The
discovery highlights the complementary nature of European and U.S.
satellites to reveal new insights about star birth and death in our
The neutron star, an ultradense ember of an exploded star, was first seen
by the European Space Agency's INTEGRAL satellite. The neutron star is in
a "double hiding place," the scientists said: It is deep in a spiral arm
of our Milky Way galaxy, obscured by dust; and it is buried in a two-star
system enshrouded by dense gas.
The scientists couldn't immediately decipher the nature of the object, so
they enlisted NASA's Rossi X-ray Timing Explorer and the newly launched
Swift satellite to observe it in different wavelengths.
"Our Galaxy's spiral arms are loaded with neutron stars, black holes and
other exotic objects," said Dr. Volker Beckmann of NASA and the
University of Maryland, Baltimore Country, lead author on a paper
appearing in the Astrophysical Journal. "The problem is, the spiral arms
are too dusty to see through. The right combination of X-ray and gamma-ray telescopes can reveal what's hiding there. And this provides
new clues about the true star formation rate in our Galaxy."
Neutron stars are the core remains of supernovas, exploded stars once
about ten times as massive as the Sun. Neutron stars contain about a
sun's worth of mass compacted into a sphere about 15 miles across. The
subject of today's announcement is a neutron star called IGR J16283-4838
in the direction of the spiral arm Norma, about 20,000 light years away.
IGR J16283-4838 is the seventh so-called "highly absorbed," or hidden,
neutron star identified. Neutron stars, born of fast-burning massive
stars, are intrinsically tied to star formation rates. They are also
energetic beacons from a region too dusty to study in detail otherwise.
As more and more are discovered, new insights about what is happening in
the Galaxy's spiral arms begin to emerge, Beckmann said.
IGR J16283-4838 revealed itself during an outburst on or near its
surface. Neutron stars such as IGR J16283-4838 are often part of binary
systems, orbiting a normal star. Occasionally, gas from the normal star,
lured by gravity, crashes onto the surface of the neutron star and
releases a great amount of energy. Outbursts can last for weeks before
the system returns to dormancy for months or years.
INTEGRAL, the Rossi Explorer and Swift each detect X rays and gamma rays,
which are far more energetic than the visible light our eyes can detect.
Yet each satellite has different capabilities. INTEGRAL has a large field
of view, enabling it to scan the Milky Way galaxy for neutron star and
black hole activity. Swift contains a high-resolution X-ray telescope,
which allowed scientists to zero in on IGR J16283-4838. The Rossi
Explorer has a timing spectrometer, a device used to uncover properties
of the light source, such as speed and rapid variations on the order of a
millisecond. The Galaxy's spiral arms block visible light from reaching
us, but not energetic X rays and gamma rays.
Simona Soldi, a doctoral candidate at INTEGRAL Science Data Centre in
Geneva discovered the new, bright source with INTEGRAL on April 7, 2005.
"We are always hunting for new sources," she said. "It's exciting when we
find something so elusive. How many more like this are out there?"
Because gamma rays are hard to focus into sharp images, the science team
used the X-ray Telescope on Swift on April 13 and 15 to determine a
precise location. Swift confirmed that the light was "highly absorbed,"
which means the binary system was filled with dense gas from the stellar
wind of the companion star. Starting on April 14, the scientists used the
Rossi Explorer to observe the source as it faded away. This observation
revealed a familiar light signature clinching the case for a fading
"high-mass X-ray binary" with a neutron star.
"Piece by piece we solved this puzzle," said Dr. Jamie Kennea of the
Swift science team at Penn State. "Swift was built primarily to detect
gamma-ray bursts, so it was thrill to use the X-ray Telescope to do
something quite different, to discover a neutron star."
INTEGRAL, the International Gamma Ray Astrophysics Laboratory, was
launched in 2002. The Rossi Explorer and Swift launched in 1995 and 2004,
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