Ultra-high-energy cosmic rays that occasionally hit the Earth may be coming from a distant source outside the Milky Way galaxy, a study suggests.
Fifty years ago, scientists discovered that the Earth is occasionally hit by cosmic rays of enormous energies.
Since then, they have argued about the source of those ultra-high-energy cosmic rays - whether they came from our galaxy or outside the Milky Way.
The answer lies in a galaxy or galaxies far, far away, according to a study published in the journal Science.
Pierre Auger Collaboration - an internationally run observatory in Argentina - has been collecting data on such cosmic rays for a more than a decade.
Researchers found that the rate of such cosmic particles, whose energies are a million times greater than that of the protons accelerated in the Large Hadron Collider (LHC), is about six per cent greater from one side of the sky than the other, in a direction where the distribution of galaxies is relatively high.
"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," said Karl- Heinz Kampert, professor at University of Wuppertal in Germany.
"Our observation provides compelling evidence that the sites of acceleration are outside the Milky Way," said Kampert, spokesperson for the Auger Collaboration, which involves more than 400 scientists from 18 countries.
Cosmic rays are the nuclei of elements from hydrogen to iron. The highest-energy cosmic rays only strike about once per square kilometre per year - equivalent to hitting the area of a soccer field about once per century.
Such rare particles are detectable because they create showers of secondary particles - including electrons, photons and muons - as they interact with the nuclei in the atmosphere.
These cosmic ray showers spread out, sweeping through the atmosphere at the speed of light in a disc-like structure, like a dinner plate but several kilometres in diameter.
After racking up detections of more than 30,000 cosmic particles, the Auger Collaboration found one section of the sky was producing significantly more than its share.
The probability of this happening by a random fluctuation is extremely small, the collaborators said, has a chance of about two in ten million.
"This result unequivocally establishes that ultra-high- energy cosmic rays are not just random wanderers of our nearby universe," said Paolo Privitera, professor at University of Chicago in the US.
"The imprint detected in their arrival directions - a tantalising evidence for extragalactic origin - required several years of observations with a detector working, in 'like a Swiss clock'," said Privitera.
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