Highest-Energy Cosmic Radiation Comes From a Mysterious Source Beyond Our Galaxy

By Ryan F. Mandelbaum on at

Not to scare you, but you’re getting hit with radiation constantly. First, there’s just regular old light (yep, that’s a kind of radiation). Then there are low levels of higher energy radiation like the kind in nuclear reactors, including particles coming out of the soil and off of bananas. But the highest-energy radiation is the weirdest stuff. It’s literally out of this galaxy.

Researchers at the Pierre Auger Observatory observed the highest energy cosmic rays, microscopic particles so energetic that you can measure them with macroscopic energy units, far more powerful than the energies created by particles in the highest-energy physics experiments. The origin of these crazy-energetic particles have long been a mystery. But scientists are getting closer to an answer. It seems these rare rays come from a preferred direction in the sky, or as physicists say, they demonstrate anisotropy.

“The result of the discovery is that the energy cannot be produced in our own galaxy,” Karl-Heinz Kampert, spokesperson of the Pierre Auger Observatory from the University in Wuppertal in Germany, told Gizmodo. “We have observational evidence that these cosmic rays come from outside our galaxy. That's a big step forward.”

The Pierre Auger Observatory includes 1,600 particle detectors in an array covering 3000 square kilometres. Each contains 12 metric tonnes of pure water that flashes a tiny amount in response to a hit from a cosmic ray, and special signal-boosting tubes that turn the flash into a signal read by a computer. The particles carry with them three important data points, said Kampert: energy, direction, and identity. This analysis, published today in the journal Science, used only the energy and direction of the particles to make its conclusion.

Image: The Pierre Auger Observatory

Counting 30,000 of these particles from 2004 to 2016, the observatory showed some strange results—on the largest scale, the particles seemed to prefer one side of the sky with a statistical-significance of 5.2 sigma. This just means that physicist’s benchmarks show the results to be extremely unlikely to have occurred due to chance. “That direction indicates an extragalactic origin for these ultrahigh-energy particles,” according to the paper, and not the center of the galaxy which some have thought could have been the source of these highest-energy cosmic rays.

At least one researcher not involved in the study, Ruth Durrer from the University of Geneva in Switzerland, thought the results were interesting. “Not very surprising, but it's nice to have this additional proof,” she said. She also added that it was clear the imbalance is not due to the motion of the earth.

The mystery of where the highest energy rays come from is far from solved, though. Magnetic fields throughout the universe bend these particles as they come in, so it’s difficult to trace them to a specific point in the sky. That means that perhaps galaxies themselves are unevenly distributed, as evidenced by the uneven distribution of the energy from the most distant light in the universe, the cosmic ray background. Or perhaps the light comes from more exotic sources.

The observation may seem to be a definitive one, but what it means is still unclear. “We are quite proud. It’s the first significant (high-sigma) observation of anisotropy of cosmic rays of these energies,” said Kampert. “It’s an important observation but now it’s up to us to interpret the data.” [Science]


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