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Source of rare cosmic rays striking Earth found to be outside the Milky Way

Published Sep 22nd, 2017 8:00PM EDT
cosmic rays
Image: NASA

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Scientists have known for a long, long time that the Earth is being hit by high-energy radiation coming from space, but in the decades since that discovery we’ve learned surprisingly little about the source. Now, in a new paper published in Science, researchers from the Pierre Auger Collaboration reveal that while they still don’t know where the cosmic rays are coming from, they know for sure where it’s not coming from: the Milky Way.

The team was able to determine the extremely rough location of the source of the radiation thanks to the habit of the rays to hit Earth from a specific angle. The extremely energetic particles slamming into us on a regular basis seem to favor one part of the sky, and that area of the sky happens to hold a large chunk of neighboring galaxies. The data used to make this determination was collected over 12 years, and the chances of it being a random statistical quirk are somewhere in the neighborhood of 1 in 5 million.

“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,” Prof. Karl-Heinz Kampert of the University of Wuppertal said. “Our observation provides compelling evidence that the sites of acceleration are outside the Milky Way.”

Unfortunately, while the discovery does provide compelling evidence that the particles aren’t originating in our own galaxy, it doesn’t exactly solve the ultimate question of what is creating them. The group plans to use upgraded observation hardware, which will be up and running in 2018, to expand on their current research and make further headway.