"Ultimate Source a Mystery" --Earth's Largest Cosmic Ray Observatory Provides Evidence of Extragalactic Origins22 Sep 2017, 16:52 UTC
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 question has now been settled by studying 30 000 cosmic-ray particles with energies a million times greater than those of the protons accelerated in the LHC . They were detected from 2004 to 2016 at the largest cosmic ray observatory ever built, the Pierre Auger Observatory in Argentina.
Analysis of the arrival directions of the particles showed that at such energies the flux of cosmic rays coming from a region of the sky located 120 degrees from the galactic center is approximately 6% higher than if the flux were perfectly uniform. This direction cannot be associated with potential sources in either the galactic plane or galactic center, providing the first convincing evidence that these cosmic rays have an extragalactic origin.
The flux of these very high-energy cosmic rays (exceeding 2 joules) is about one particle per ...