Cosmic Rays Arriving on Earth from Outside the Milky Way

Cosmic Rays, Milky Way

The discovery solves an astronomical mystery of more than half a century ago, but does not reveal the sources that originate these energetic particles.

Since the existence of extremely energetic cosmic rays in the 1960s, there has been much speculation as to whether these particles come from our own galaxy as well as from much more distant places.

Now the group of more than 400 scientists from 18 countries that make up the collaboration of the Pierre Auger Observatory reveals for the first time that the very high energy cosmic rays that reach the Earth originate outside the Milky Way, as published in Science.

After 12 years of data collection, the extragalactic origin of this type of lightning has been possible thanks to the detection of particles with an average energy of 2 Joule at the Pierre Auger Observatory located in Argentina and to detect an asymmetry in its distribution: the amount arriving in one direction, 120 degrees away from the center of our galaxy, is 6% greater than in the opposite direction.

Karl-Heinz-Kampert, a professor at the University of Wuppertal (Germany) and spokesman for the Auger Collaboration, stresses the importance of the finding: “We are now considerably closer to solving the mystery of where and how these extraordinary energy particles are created, a question of enormous interest for astrophysics.

Our observations represent compelling evidence that the places where they accelerate are beyond the Milky Way. “

“There has been other evidence, but I would say that this article really confirms that most of the higher-energy cosmic ray particles do not come from our galaxy,” underscores Gregory Snow, a professor of physics at the University of Nebraska-Lincoln and a participant in the project.

Professor Alan Watson of the University of Leeds (United Kingdom), spokesman emeritus, believes that this result “is one of the most exciting that we have obtained, and responds to one of the key questions that it was intended to answer when it was conceived the observatory by Jim Cronin and myself over 25 years ago. “

Cosmic rays are atomic nuclei of different elements, from lighter ones like hydrogen, with only a proton, to heavier ones like iron.

At high energies, over 2 Joule (1 Joule equals to approximately 6 x 1018 electron volts), their rate of arrival on Earth decreases so much that they cross a surface equivalent to a football field with an average frequency of once per century.

Even so, being so rare these cosmic rays are detected because, in successive interactions with the nuclei of the atmosphere, they produce avalanches or ‘rains’ of multiple electrons, protons and neutrons that plow the atmosphere practically at the speed of light, grouped in the shape of a disk, like a flat plate several kilometers in diameter.

Rains of Millions of Particles

These ‘rains’, which contain more than ten billion particles, are detected because they produce a shock wave of light in the water (Cherenkov light) as they pass through some of the 1,600 Auger Observatory detectors, each with 12 tons of water, which are scattered in 3,000 square kilometers to the west of Argentina, on an area comparable to the island of Mallorca.

The arrival times of these particles in the detectors, measured with GPS receivers, are used to establish the direction of arrival of the cosmic ray with an accuracy better than 1 degree.

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