GALAXY PICTURE: Cosmic Ray Mystery Solved?

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.

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."

Photograph courtesy NASA, ESA, CXC, and JPL-Caltech