NASA'S Fermi Proves Supernova Remnants Produce Cosmic Rays

Feb. 14, 2013

J. D. Harrington
Headquarters, Washington
202-358-5241
j.d.harrington@nasa.gov

Lynn Chandler
NASA Goddard Space Flight Center, Greenbelt, Md.
301-286-2806
lynn.chandler-1@nasa.gov



RELEASE: 13-052

NASA'S FERMI PROVES SUPERNOVA REMNANTS PRODUCE COSMIC RAYS

WASHINGTON -- A new study using observations from NASA's Fermi
Gamma-ray Space Telescope reveals the first clear-cut evidence the
expanding debris of exploded stars produces some of the
fastest-moving matter in the universe. This discovery is a major step
toward understanding the origin of cosmic rays, one of Fermi's
primary mission goals.

"Scientists have been trying to find the sources of high-energy cosmic
rays since their discovery a century ago," said Elizabeth Hays, a
member of the research team and Fermi deputy project scientist at
NASA's Goddard Space Flight Center in Greenbelt, Md. "Now we have
conclusive proof supernova remnants, long the prime suspects, really
do accelerate cosmic rays to incredible speeds."

Cosmic rays are subatomic particles that move through space at almost
the speed of light. About 90 percent of them are protons, with the
remainder consisting of electrons and atomic nuclei. In their journey
across the galaxy, the electrically charged particles are deflected
by magnetic fields. This scrambles their paths and makes it
impossible to trace their origins directly.

Through a variety of mechanisms, these speedy particles can lead to
the emission of gamma rays, the most powerful form of light and a
signal that travels to us directly from its sources.

Since its launch in 2008, Fermi's Large Area Telescope (LAT) has
mapped million- to billion-electron-volt (MeV to GeV) gamma-rays from
supernova remnants. For comparison, the energy of visible light is
between 2 and 3 electron volts.

The Fermi results concern two particular supernova remnants, known as
IC 443 and W44, which scientists studied to prove supernova remnants
produce cosmic rays. IC 443 and W44 are expanding into cold, dense
clouds of interstellar gas. These clouds emit gamma rays when struck
by high-speed particles escaping the remnants.

Scientists previously could not determine which atomic particles are
responsible for emissions from the interstellar gas clouds because
cosmic ray protons and electrons give rise to gamma rays with similar
energies. After analyzing four years of data, Fermi scientists see a
distinguishable feature in the gamma-ray emission of both remnants.
The feature is caused by a short-lived particle called a neutral
pion, which is produced when cosmic ray protons smash into normal
protons. The pion quickly decays into a pair of gamma rays, emission
that exhibits a swift and characteristic decline at lower energies.
The low-end cutoff acts as a fingerprint, providing clear proof that
the culprits in IC 443 and W44 are protons.

The findings will appear in Friday's issue of the journal Science.

"The discovery is the smoking gun that these two supernova remnants
are producing accelerated protons," said lead researcher Stefan Funk,
an astrophysicist with the Kavli Institute for Particle Astrophysics
and Cosmology at Stanford University in Calif. "Now we can work to
better understand how they manage this feat and determine if the
process is common to all remnants where we see gamma-ray emission."

In 1949, the Fermi telescope's namesake, physicist Enrico Fermi,
suggested the highest-energy cosmic rays were accelerated in the
magnetic fields of interstellar gas clouds. In the decades that
followed, astronomers showed supernova remnants were the galaxy's
best candidate sites for this process.

A charged particle trapped in a supernova remnant's magnetic field
moves randomly throughout the field and occasionally crosses through
the explosion's leading shock wave. Each round trip through the shock
ramps up the particle's speed by about 1 percent. After many
crossings, the particle obtains enough energy to break free and
escape into the galaxy as a newborn cosmic ray.

The supernova remnant IC 443, popularly known as the Jellyfish Nebula,
is located 5,000 light-years away toward the constellation Gemini and
is thought to be about 10,000 years old. W44 lies about 9,500
light-years away toward the constellation Aquila and is estimated to
be 20,000 years old. Each is the expanding shock wave and debris
formed when a massive star exploded.

The Fermi discovery builds on a strong hint of neutral pion decay in
W44 observed by the Italian Space Agency's AGILE gamma ray
observatory and published in late 2011.

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

For images and a video related to this finding, please visit:

http://go.nasa.gov/Yp14cJ

For more information about NASA's Fermi Gamma-ray Space Telescope and
its mission, visit:

http://www.nasa.gov/fermi


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