Cosmic rays from galaxies far far away

Cosmic rays with energies a million times greater than the protons accelerated in the Large Hadron Collider are coming from far outside our galaxy, say scientists from the Pierre Auger Collaboration.

Ever since the existence of cosmic rays with individual energies of several Joules[1] was established in the 1960s, speculation has raged as to whether such particles are created in our galaxy or in distant extragalactic objects. The 50-year-old mystery has been solved using cosmic particles with a mean energy of 2 Joules recorded by the Pierre Auger Observatory in Argentina — the largest cosmic-ray observatory ever built. 

The observations, published today in Science, have found that at a mean energy of 2 Joules, the cosmic rays’ rate of arrival is ~6% greater from one half of the sky, with the excess lying 120˚ away from the Galactic centre.

Spokesperson for the Pierre Auger Collaboration, Professor Karl-Heinz Kampert from the University of Wuppertal in Germany, said: "We are now considerably closer to solving the mystery of where and how these extraordinary particles are created, a question of great interest to astrophysicists. Our observation provides compelling evidence that the sites of acceleration are outside the Milky Way.”

Emeritus spokesperson for the Pierre Auger Collaboration, Emeritus Professor Alan Watson from the School of Physics and Astronomy at the University of Leeds said: “I consider this to be one of the most exciting results that we have obtained and one which solves a problem targeted when the Observatory was conceived by Jim Cronin and myself over 25 years ago”.

Cosmic rays are the nuclei of elements from hydrogen (the proton) to iron. Above 2 Joules, the rate of their arrival at the top of the atmosphere is only about one per square kilometre per year — equivalent to one hitting the area of a football pitch about once per century. Such rare particles are detectable because they create showers of electrons, photons and muons through successive interactions with the nuclei in the atmosphere.  These showers spread out, sweeping through the atmosphere at the speed of light in a disc-like structure, similar to a dinner-plate, several kilometres in diameter. 

The showers contain more than ten billion particles and are detected at the Pierre Auger Observatory through the Cherenkov light they produce in a few of the 1,600 detectors spread over 3,000 km2 of Western Argentina — an area comparable to that inside the M25 around London.  The times of arrival of the particles at the detectors, measured with GPS receivers, are used to find the arrival directions of events to within ~1˚. 

By studying the distribution of the arrival directions of more than 30,000 cosmic particles, the Pierre Auger Collaboration has discovered an anisotropy, significant at 5.2 standard deviations (a chance of about two in ten million), in a direction where the distribution of galaxies is relatively high.

Although this discovery clearly indicates an extragalactic origin for the particles, the actual sources have yet to be pinned down.  The direction of the excess points to a broad area of sky rather than to specific sources as the magnetic field of our galaxy causes even particles as energetic as these to be deflected by a few 10s of degrees. The direction, however, cannot be associated with putative sources in the plane or centre of our galaxy for any realistic configuration of the galactic magnetic field.