GALAXY PICTURE: Cosmic Ray Mystery Solved?

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November 2, 2009—Star explosions can act like ultra-powerful particle colliders in space, creating what are known as cosmic rays, new research shows.

Cosmic rays, which are actually high-energy subatomic particles, are constantly bombarding Earth. The most intense of these tiny particles can hit with the same force as a 98-mile-an-hour (157-kilometer-an-hour) fastball. (Related: "Ancient Mass Extinctions Caused by Cosmic Radiation, Scientists Say.")

Astronomers know that cosmic rays come from far away in the galaxy. But the magnetic fields of larger objects such as planets and stars bend the particles' flight paths, making it hard to trace their exact sources.

In addition, the magnetic field of the galaxy itself traps cosmic rays inside, sending them darting about like flies in a sealed mason jar.

Some astronomers have suggested that cosmic rays might come from supernova remnants. When a massive star explodes, the theory goes, the expanding shock wave pulls along charged particles. These particles bounce around inside the supernova remnant's magnetic field until they reach near-light speed and escape into the galaxy as cosmic rays.

Until now this idea has been hard to test, since we can't trace the Milky Way's cosmic rays and we can't see the cosmic rays trapped inside other galaxies. Now, for the first time, an international team using the Very Energetic Radiation Imaging Telescope Array System (VERITAS) and the Fermi Gamma-ray Space Telescope has found strong support for the supernova theory.

So-called starburst galaxies should have more cosmic rays than "normal" galaxies like our Milky Way, according to the theory. That's because such galaxies have regions of rapid star formation, which give rise to more of the supermassive stars that end their lives in supernova explosions.

For their study, the team searched for gamma rays, the most energetic form of light.

Unlike cosmic rays, light isn't affected by magnetic fields, so we can see it from Earth and accurately trace its source.

"We believe [the] gamma rays are coming from cosmic rays interacting with the interstellar medium," team member Keith Bechtol, of the Kavli Institute for Particle Astrophysics and Cosmology in Stanford, California, said today during a press briefing.

As expected, the VERITAS team found higher amounts of gamma rays coming from the starburst galaxy M82 (pictured), about 12 million light-years from Earth. The Fermi probe also saw gamma rays coming from M82 and from the starburst galaxy NGC 253. In addition, Fermi saw gamma rays coming from a star-forming region in the Large Magellanic Cloud, a small satellite of the Milky Way.

"Galaxies with more supernovae should be more gamma ray bright, and that's what we're finding," said team member Charles Dermer, of the U.S. Naval Research Laboratory in Washington, D.C.

But this process creates cosmic rays only up to a certain energy level. The most energetic cosmic rays likely come from jets of particles being belched out by supermassive black holes, although that theory has yet to be tested.

Still, said team member Jürgen Knödlseder of the Centre dEtude Spatiale des Rayonnements in France, the new discovery is "one more piece of the big puzzle in understanding where cosmic rays come from."

—Victoria Jaggard

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