Observatory detects extragalactic cosmic rays hitting the Earth

Scienmag  October 24, 2017

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 is a galaxy or galaxies far, far away, according to a report published Sept. 22 in Science by the Pierre Auger Collaboration. The internationally run observatory in Argentina, co-founded by the late University of Chicago Nobel laureate James Cronin, has been collecting data on such cosmic rays for a more than a decade.

The collaboration 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, is about six percent 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 University of Wuppertal Prof. Karl-Heinz Kampert, spokesperson for the Auger Collaboration, which involves more than 400 scientists from 18 countries. "Our observation provides compelling evidence that the sites of acceleration are outside the Milky Way."