Scientists discover mysterious source of high-energy cosmic rays

Scientists on Wednesday announced the discovery of a previously unidentified nearby source of high-energy cosmic rays. The finding was made with a NASA-funded balloon-borne instrument high over Antarctica.

Researchers from the Advanced Thin Ionization Calorimeter (ATIC) collaboration, led by scientists at Louisiana State University, Baton Rouge, published the results in the 20 November issue of the journal `Nature'. The new results show an unexpected surplus of cosmic ray electrons at very high energy - 300-800 billion electron volts - that must come from a previously unidentified source or from the annihilation of very exotic theoretical particles used to explain dark matter.

"This electron excess cannot be explained by the standard model of cosmic ray origin," said John P Wefel, ATIC project principal investigator and a professor at Louisiana State. "There must be another source relatively near us that is producing these additional particles."

Explaining the rarity of this find, Wefel said, ''This is a big discovery. It's the first time we've seen a discrete source of accelerated cosmic rays standing out from the general galactic background.''

According to the research, this source would need to be within about 3,000 light years of the sun. It could be an exotic object such as a pulsar, mini-quasar, supernova remnant or an intermediate mass black hole.

Galactic cosmic rays are subatomic particles accelerated to almost light speed by distant supernova explosions and other violent events. They swarm through the Milky Way, forming a haze of high-energy particles that enter the solar system from all directions. Cosmic rays consist mostly of protons and heavier atomic nuclei with a dash of electrons and photons spicing the mix.

To study the most powerful and interesting cosmic rays, Wefel and colleagues have spent the last eight years flying a series of balloons through the stratosphere over Antarctica. Each time the payload was a NASA-funded cosmic ray detector named ATIC, short for Advanced Thin Ionization Calorimeter. The team expected ATIC to tally the usual mix of particles, mainly protons and ions, but the calorimeter found something extra: an abundance of high-energy electrons.

"Cosmic ray electrons lose energy during their journey through the galaxy," said Jim Adams, ATIC research lead at NASA's Marshall Space Flight Center in Huntsville, Ala. "These losses increase with the energy of the electrons. At the energies measured by our instrument, these energy losses suppress the flow of particles from distant sources, which helps nearby sources stand out."

However, the scientists point out, there are few such objects close to our solar system. "These results may be the first indication of a very interesting object near our solar system waiting to be studied by other instruments," Wefel said.

The least exotic possibilities may include a nearby pulsar, a 'microquasar' or a stellar-mass black hole - all are capable of accelerating electrons to these energies. It is possible that such a source lurks undetected not far away. NASA's recently launched Fermi Gamma-ray Space Telescope is only just beginning to survey the sky with sufficient sensitivity to reveal some of these objects.

An alternative explanation is that the surplus of high-energy electrons might result from the annihilation of very exotic particles put forward to explain dark matter. In recent decades, scientists have learned that the kind of material making up the universe around us only accounts for about five per cent of its mass composition. Close to 70 per cent of the universe is composed of dark energy (so called because its nature is unknown). The remaining 25 per cent of the mass acts gravitationally just like regular matter, but does little else, so it is normally not visible.

There is a class of physical theories called "Kaluza-Klein theories" which seek to reconcile gravity with other fundamental forces by positing extra dimensions. In addition to the familiar 3D of human experience, there could be as many as eight more dimensions woven into the space around us. A popular yet unproven explanation for dark matter is that dark matter particles inhabit the extra dimensions. We feel their presence via the force of gravity, but do not sense them in any other way.

How does these produce excess cosmic rays? Kaluza-Klein particles have the curious property (one of many) that they are their own anti-particle. When two collide, they annihilate one another, producing a spray of high-energy photons and electrons. The electrons are not lost in hidden dimensions, however, they materialize in the 3-dimensions of the real world where ATIC can detect them as "cosmic rays."

"The annihilation of these exotic particles with each other would produce normal particles such as electrons, positrons, protons and antiprotons that can be observed by scientists," said Eun-Suk Seo, ATIC lead at the University of Maryland, College Park.

ATIC is an international collaboration of researchers from Louisiana State University, the University of Maryland, Marshall Space Flight Center, Purple Mountain Observatory in China, Moscow State University in Russia and the Max-Planck Institute for Solar System Research in Germany. ATIC is supported in the US by NASA and flights are conducted under the auspices of the Balloon Program Office at NASA's Wallops Flight Facility in Virginia by the staff of the Columbia Scientific Balloon Facility. The National Science Foundation provides antarctic logistics.