Researchers have found that some ultra-high energy cosmic rays that occasionally hit Earth come from a distant source outside the Milky Way.
Cosmic rays are atomic nuclei that travel through space at speeds close to that of light. Low-energy cosmic rays come from the Sun or from our own galaxy, but the origin of the highest-energy particles has been the subject of debate ever since they were first discovered fifty years ago.
Do they come from our Galaxy or from distant extragalactic objects?
The study published in the journal Science demonstrated that those cosmic rays with energies a million times greater than that of the protons accelerated in the Large Hadron Collider - the world's largest and most powerful particle accelerator - come from much further away than from our own galaxy.
They were detected from 2004 to 2016 at the largest cosmic ray observatory ever built, the Pierre Auger Observatory in Argentina.
"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 from University of Wuppertal in Germany.
"Our observation provides compelling evidence that the sites of acceleration are outside the Milky Way," Kampert who is spokesperson for the Auger Collaboration, which involves more than 400 scientists from 18 countries, said.
Cosmic rays are the nuclei of elements from hydrogen to iron. The highest-energy cosmic rays, those of interest in this study, 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.
At the Auger Observatory, the shower particles are detected through the light they produce in several of 1,600 detectors, spread over 3,000 square kilometres of western Argentina and each containing 12 tons of water.
Tracking these arrivals tells scientists the direction from which the cosmic rays came.
After racking up detections of more than 30,000 cosmic particles, the scientists 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 scientists said -- 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," Paolo Privitera of University of Chicago who heads the US groups participating in the project, said.