Cosmic Accelerators Discovered in Our Galaxy --Alexander Kusenko--

August 6, 2010
Institute for the Physics and Mathematics of the Universe (IPMU)

Analyzing the data from the largest cosmic ray detector in history, scientists from UCLA and Japan have discovered evidence of natural nuclear accelerators at work in our own Milky Way galaxy.

Cosmic rays of the highest energies were believed to come from remote galaxies hosting gigantic black holes capable of consuming stars and accelerating protons to "macroscopic" energies, comparable to that of a bullet from a modern rifle. However, earlier this year, Pierre Auger Observatory has published a surprising discovery: many of the energetic cosmic rays are, in fact, nuclei, not protons. And the higher the energy, the more nuclei per proton the observers detect. This was totally unexpected because nuclei, more fragile than protons, tend to disintegrate into protons on their long journey through space. Moreover, it is very unlikely that a cosmic accelerator of any kind would accelerate nuclei better than protons at these high energies.

The resolution of the paradox came from the analysis of Calvez, Kusenko, and Nagataki, to be published in an upcoming issue of Physical Review Letters.
Stellar explosions in our own Galaxy can accelerate both protons and nuclei, but, while the protons leave the Galaxy promptly, the heavier and less mobile nuclei get trapped in the turbulent magnetic field and linger longer than protons. As a result, the local density of nuclei is increased, and they bombard Earth in greater numbers, as seen by the Pierre Auger Observatory.

Stellar explosions capable of accelerating particles to ultrahigh energies have been seen in other galaxies, where they produce gamma-ray bursts. The new analysis provides evidence that such powerful explosions took place in our Galaxy as well, at least a few times per million years.

The ultrahigh-energy nuclei observed today have been trapped in the web of Galactic magnetic fields for millions of years, and their arrival directions have been completely randomized by the numerous twists and turns in the tangled field. However, the researchers predict, the protons escaping from other galaxies should still be seen at the highest energies and should point back to their sources, providing Pierre Auger Observatory with valuable data for charged-particle astronomy.

Publication : Physical Review Letters  2010 August 20 issue

Title: Role of Galactic sources and magnetic fields in forming the observed energy-dependent composition of ultrahigh-energy cosmic rays

Authors: Antoine Calvez (UCLA), Alexander Kusenko (UCLA/IPMU) and Shigehiro Nagataki (Kyoto University)

Preprint URL:


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UCLA Professor

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