Vast Cloud of Antimatter Discovered Near Center of Milky Way

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Several years of observations from the European Space Agency’s
Integral (INTErnational Gamma-Ray Astrophysics Laboratory) satellite have solved one of the most vexing mysteries in our Milky Way:
the origin of a giant cloud of antimatter surrounding the galactic
center.

The shape of the mysterious cloud of antimatter in the central regions
of the Milky Way has been revealed by Integral. The unexpectedly lopsided shape is a new clue to
the origin of the antimatter. The observations have debunked the chances that the antimatter is
coming from the annihilation or decay of astronomical dark matter.

he cloud extends farther on the western side of the galactic center
than it does on the eastern side. This imbalance matches the
distribution of a population of binary star systems that contain black
holes or neutron stars, strongly suggesting that these binaries are
churning out at least half of the antimatter, and perhaps all of it.

The
cloud itself is roughly 10,000 light-years across, and generates the
energy of about 10,000 Suns. The cloud shines brightly in gamma rays
due to a reaction governed by Einstein’s famous equation E=mc^2.
Negatively charged subatomic particles known as electrons collide with
their antimatter counterparts, positively charged positrons. When
electrons and positrons meet, they can annihilate one another and
convert all of their mass into gamma rays with energies of 511,000
electron-volts (511 keV).

The
antimatter cloud was discovered in the 1970s by gamma-ray detectors
flown on balloons. Scientists have proposed a wide range of
explanations for the origin of the antimatter, which is exceedingly
rare in the cosmos. For years, many theories centered around
radioactive elements produced in supernovae, prodigious stellar
explosions. Others suggested that the positrons come from neutron
stars, novae, or colliding stellar winds.

In recent years, some
theorists championed the idea that particles of dark matter were
annihilating one another, or with atomic matter, producing electrons
and positrons that annihilate into 511-keV gamma rays. But other
scientists remained skeptical, noting that the dark matter particles
had to be significantly lighter than most theories predicted.

"The
Integral results seem to rule out dark matter as the major source of
the gamma rays," said Gerry Skinner,
who currently works at NASA’s Goddard Space Flight Center in Greenbelt,
Md., Skinner is a co-investigator of Integral’s SPI (SPectrometer for
Integral) instrument, which made this discovery.

Integral found certain types of binary systems near
the galactic center are also skewed to the west. These systems are
known as hard low-mass X-ray binaries, since they light up in
high-energy (hard) X-rays as gas from a low-mass star spirals into a
companion black hole or neutron star. Because the two "pictures" of
antimatter and hard low-mass X-ray binaries line up strongly suggests
the binaries are producing significant amounts of positrons.

"Simple
estimates suggest that about half and possibly all the antimatter is
coming from X-ray binaries," says Georg Weidenspointer of the Max
Planck Institute for Extraterrestrial Physics in Germany.

While Integral’s discovery clears up one
mystery, it raises a new one. Scientists don’t understand how low-mass
X-ray binaries could produce enough positrons to explain the cloud, and
they also don’t know how they escape from these systems. "We expected
something unexpected, but we did not expect this," says Skinner. The
antimatter is probably produced in a region near the neutron stars and
black holes, where powerful magnetic fields launch jets of particles
that rip through space at near-light speed.

NASA’s Gamma-ray
Large Area Space Telescope (GLAST), scheduled to launch in 2008, may
help clarify how objects such as black holes launch particle jets.
Conceivably, it could even detect higher-energy gamma rays from heavier
types of dark matter particles annihilating one another.

Launched
in 2002, Integral is an international mission involving scientists and
engineers from dozens of nations. NASA contributed scientists,
hardware, and software for Integral, including part of the SPI
instrument.

The new results give astronomers a valuable new clue and point away
from dark matter as the origin of the antimatter. Beyond the Milk Way's
center, the cloud is not entirely spherical. Instead it is lopsided
with twice as much on one side of the galactic center as the other.
Such a distribution is unusual because gas in the inner region of the
galaxy is relatively evenly distributed.

Equally importantly, Integral found evidence that a population of
binary stars is also significantly off-center, corresponding in extent
to the cloud of antimatter. That powerfully suggests these objects,
known as hard (because they emit at high energies) low mass X-ray
binaries, are responsible for a major amount of antimatter.

The researchers calculate that a relatively ordinary star getting
torn apart by a black hole or neutron star orbiting around it — a
so-called "low mass X-ray binary" — could spew on the order of one
hundred thousand billion billion billion billion positrons (a 1
followed by 41 zeros) per second. These could account for a great deal
of the antimatter that scientists have inferred, reducing or
potentially eliminating the need for exotic explanations such as ones
involving dark matter.

“Simple estimates suggest that about half and possibly all of the
antimatter is coming from the X-ray binaries,” says Weidenspointner.
The other half could be coming from a similar process around the
galaxy’s central black hole and the various exploding stars there.

The discovery has real astrophysical importance because it decreases the need for dark matter at the center of our galaxy.

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Posted by Casey Kazan.

Links:

http://www.esa.int/esaCP/SEMKTX2MDAF_index_0.html

http://www.nasa.gov/centers/goddard/news/topstory/2007/antimatter_binary.html

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