Point Sources for Extreme Cosmic Rays?

The 1930s and '40s was a golden age for cosmic rays; positrons, pions, and muons were first discovered in those years not at accelerators but in emulsions exposed to incoming cosmic ray showers.

It could be argued that we now live in another cosmic ray golden age: fields of detectors spread out over many square miles on the Earth's surface have piled up an impressive bank of events with reconstructed shower energies stretching above 10²⁰ eV.

But how does the great accelerator in the sky work? Are the highest energy cosmic rays coming from inside or outside the Milky Way? Can we pinpoint specific sources or is the generation of the rays amorphous? Why are the energies so high?

A session at last week's AAS/APS meeting in Albuquerque addressed these issues. Masahiro Teshima of the University of Tokyo reported on a study of 59 events with energies greater than 4*10¹⁹ eV, as recorded by the Akeno Giant Air Shower Array (AGASA) in Japan.

Teshima pointed to a modest but unmistakable clustering in the form of five doublet and even one triplet alignment. That is, in five cases pairs of energetic events had come from the same place in the sky, while in once case three different events originated at the same place. No identification has yet been made of specific celestial objects with those coordinates.

Teshima said that the number of doublets or triplets might be even higher if one could properly model the Milky Way's magnetic field (the galactic equivalent of Earth's magnetosphere) and thus take into account how the trajectories of cosmic rays were distorted on their way toward Earth.