University of California, Los Angeles | 17 Aug 2010
Role of Galactic sources and magnetic fields in forming the observedPhysicists from UCLA and Japan have discovered evidence of "natural nuclear accelerators" at work in our Milky Way galaxy, based on an analysis of data from the world's largest cosmic ray detector.
The research is published Aug. 20 in the journal Physical Review Letters.
Cosmic rays of the highest energies were believed by physicists to come from remote galaxies containing enormous black holes capable of consuming stars and accelerating protons at energies comparable to that of a bullet shot from a rifle. These protons — referred to individually as "cosmic rays" — travel through space and eventually enter our galaxy.
But earlier this year, physicists using the Pierre Auger Observatory in Argentina, the world's largest cosmic ray observatory, published a surprising discovery: Many of the energetic cosmic rays found in the Milky Way are not actually protons but nuclei — and the higher the energy, the greater the nuclei-to-proton ratio.
"This finding was totally unexpected because the nuclei, more fragile than protons, tend to disintegrate into protons on their long journey through space," said Alexander Kusenko, UCLA professor of physics and astronomy and co-author of the Physical Review Letters research. "Moreover, it is very unlikely that a cosmic accelerator of any kind would accelerate nuclei better than protons at these high energies."
The resolution to the paradox of the nuclei's origin comes from an analysis by Kusenko; Antoine Calvez, a UCLA graduate student of physics who is part of Kusenko's research group; and Shigehiro Nagataki, an associate professor of physics at Japan's Kyoto University. They found that stellar explosions in our own galaxy can accelerate both protons and nuclei. But while the protons promptly leave the galaxy, the heavier and less mobile nuclei become trapped in the turbulent magnetic field and linger longer.
energy-dependent composition of ultrahigh-energy cosmic rays - A Calvez et al
- arXiv.org > astro-ph > arXiv:1004.2535 > 15 Apr 2010 (v1), 04 Aug 2010 (v3)
Physical Review Letters (accepted 26 July 2010)
New Scientist | Cosmology | 17 Aug 2010
A portion of the high-energy cosmic rays raining down on Earth could have been swirling around the Milky Way for millions of years. The idea could help solve a conundrum that has puzzled physicists for the past year.
Cosmic rays are protons and nuclei of varying energies that regularly strike Earth. Little is known about them, but it is assumed that those with higher energies should have lower masses, as they are thought more likely to break up en route to Earth.
It was therefore a surprise when the Pierre Auger Observatory in Argentina last year reported that the average mass of cosmic rays was increasing at high energies.
Now a team led by Alexander Kusenko of the University of California, Los Angeles, have an explanation.