NASA's Fermi explores high-energy "space invaders"

WASHINGTON, May 4 (Xinhua) -- Since its launch last
June, NASA's Fermi Gamma-ray Space Telescope has discovered a new class of
pulsars, probed gamma-ray bursts and watched flaring jets in galaxies billions
of light-years away. On Monday at the American Physical Society meeting in
Denver, Colorado, Fermi scientists revealed new details about high-energy
particles implicated in a nearby cosmic mystery.

"Fermi's Large Area Telescope is a state-of-the-art
gamma-ray detector, but it's also a terrific tool for investigating the
high-energy electrons in cosmic rays," said Alexander Moiseev, who presented the
findings. Moiseev is an astrophysicist at NASA's Goddard Space Flight Center in
Greenbelt, Maryland.

Cosmic rays are hyper fast electrons, positrons, and
atomic nuclei moving at nearly the speed of light. Astronomers believe that the
highest-energy cosmic rays arise from exotic places within our galaxy, such as
the wreckage of exploded stars.

Fermi's Large Area Telescope (LAT) is exquisitely
sensitive to electrons and their antimatter counterparts, positrons. Looking at
the energies of 4.5 million high-energy particles that struck the detector
between Aug. 4, 2008 and Jan. 31, 2009, the LAT team found evidence that both
supplements and refutes other recent findings.

Compared to the number of cosmic rays at lower
energies, more particles striking the LAT had energies greater than 100 billion
electron volts (100 GeV) than expected based on previous experiments and
traditional models. (Visible light has energies between two and three electron
volts.) The observation has implications similar to complementary measurements
from a European satellite named PAMELA and from the ground-based High Energy
Stereoscopic System (H.E.S.S.), an array of telescopes located in Namibia that
sees flashes of light as cosmic rays strike the upper atmosphere.

Last fall, a balloon-borne experiment named ATIC
captured evidence for a dramatic spike in the number of cosmic rays at energies
around 500 GeV. "Fermi would have seen this sharp feature if it was really
there, but it didn't," said Luca Latronico, a team member at the National
Institute of Nuclear Physics (INFN) in Pisa, Italy. "With the LAT's superior
resolution and more than 100times the number of electrons collected by
balloon-borne experiments, we are seeing these cosmic rays with unprecedented
accuracy."

Unlike gamma rays, which travel from their sources in
straight lines, cosmic rays wend their way around the galaxy. They can ricochet
off of galactic gas atoms or become whipped up and redirected by magnetic
fields. These events randomize the particle paths and make it difficult to tell
where they originated. In fact, determining cosmic-ray sources is one of Fermi's
key goals.

What's most exciting about the Fermi, PAMELA, and
H.E.S.S. data is that they may imply the presence of a nearby object that's
beaming cosmic rays our way. "If these particles were emitted far away, they'd
have lost a lot of their energy by the time they reached us," explained Luca
Baldini, another Fermi collaborator at INFN.

If a nearby source is sending electrons and positrons
toward us, the likely culprit is a pulsar -- the crushed, fast-spinning leftover
of an exploded star. A more exotic possibility is on the table, too. The
particles could arise from the annihilation of hypothetical particles that
make-up so-called dark matter. This mysterious substance neither produces nor
impedes light and reveals itself only by its gravitational effects.

"Fermi's next step is to look for changes in the
cosmic-ray electron flux in different parts of the sky," Latronico said. "If
there is a nearby source, that search will help us unravel where to begin
looking for it."

NASA's Fermi Gamma-ray Space Telescope is an
astrophysics and particle physics partnership mission, developed in
collaboration with the U.S. Department of Energy and important contributions
from academic institutions and partners in France, Germany, Italy, Japan,
Sweden, and the United States.