Cosmic Rays Originate Far Beyond the Milky Way

Scientists have hypothesized that cosmic rays might originate in the Milky Way, but a dozen years of observational data points to extragalactic origins.

Something out in space has been bombarding Earth with incredibly high energy particles called cosmic rays. The origin of cosmic rays has been a mystery since their discovery over a century ago.

But twelve years of data from an unusual observatory in South America has now confirmed that cosmic rays with the highest energies come from sources outside the Milky Way. In particular, the majority of the high-energy particles originate from an area of the sky that lies almost opposite from the center of our own galaxy, in a region of space with a high concentration of other galaxies.  

“The distribution of arrival directions of the highest energy cosmic ray particles has an enhancement in a broad patch of the sky which is roughly 120 degrees away from a line pointing from Earth to the center of our Milky Way galaxy, meaning cosmic rays coming to the Earth from that patch must be coming from other galaxies,” Gregory Snow, a physics professor from the University of Nebraska-Lincoln, said in an email to Seeker. He is also the education and outreach coordinator for the Pierre Auger Observatory, which is located in western Argentina and was the source of the data.

Snow and a group of more than 400 scientists from 18 countries published last week their analysis of cosmic rays in the journal Science.

He explained the direction of the enhanced patch is consistent with a region of galaxies that is more dense than other regions of the sky.

“This makes sense since we might expect more cosmic ray particles coming from places in the universe where there is a lot of ‘stuff,’” he said.

Cosmic rays are electrically charged particles, such as protons, that travel near the speed of light. They strike Earth from all directions, and have energies up to one hundred million times higher than those created in man-made accelerators like the Large Hadron Collider. These high-energy cosmic rays — ones with energies reaching quintillions of electron volts — are different from the ones emitted by our sun during flares and coronal mass ejections.

"The sun emits low-energy cosmic ray particles that are detected here on Earth, but they are nowhere near as high energy as the particles detected at the Auger Observatory," Snow explained in a press release.

When the high energy cosmic rays travel across space, the particles can be deflected by magnetic fields, which scramble their paths and sometimes mask their origins.

Detecting cosmic rays is even more challenging because the highest energy particles — the ones that are most mysterious and rare — reach Earth at a rate of only one particle per square kilometer each year.

That’s where the Pierre Auger Observatory comes in. The observatory uses 1,660 tanks filled with ultra-pure water, spread over a 1,800-square-mile (3,000-square-kilometer) grid in Argentina. Each 3,000-gallon (12,000 liter) tank is separated from the other tanks by about a mile (1.5 km) and are enclosed to make them completely dark inside. When cosmic ray particles pass through the water, their electromagnetic shock waves produce radiation called Cherenkov light that can be measured by special instruments mounted in the tanks.

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There are also separate, independent detectors called air fluorescence telescopes that track the development of what is called “air showers.” Cosmic rays interacting with Earth’s atmosphere produce a cascade effect, creating extensive showers that contain billions of secondary particles. The air fluorescence telescopes observe ultraviolet light emitted high in Earth's atmosphere from the showers.  These air showers can also cause nearly simultaneous bursts of light in more than five tanks.

Using the two detectors, scientists can determine the energy of the primary cosmic ray particles based on the amount of light they detect from a sample of secondary particles. Additionally, slight differences in the detection times at different tank positions help scientists determine the trajectory of the incoming cosmic rays.

In over a dozen years of operation, the Auger Observatory has collected some of the highest quality information about the types of particles in primary cosmic rays. Comparing results from the different types of detectors also helps scientists reconcile the two sets of data and produce the most accurate results about the energy of primary cosmic rays.

But why study these high-energy cosmic rays?

For one thing, scientists like a good mystery and the origins of cosmic rays is one of the biggest unknowns in physics.

But understanding them better could lead to improved insights on fundamental physics, such as how our universe was created, and why objects have mass. Snow told Seeker high-energy cosmic rays are clues to the very structure of the universe.

“High-energy cosmic ray particles are one of several messengers from outer space that we use to learn about the structure of the universe, for example, the distribution of where the billions of other galaxies apart from the Milky Way are located,” he said. “We now know that galaxies are not uniformly distributed in outer space. Rather they group themselves in clusters and super-clusters.”

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Also, scientists don’t know the exact source of high-energy cosmic rays. There have been theories, but the intense conditions needed to generate such energetic particles can be mind-boggling.

“We know that shock waves coming from stars dying in the form of a supernova could accelerate cosmic ray particles up to energies reaching about 10 to the 15th electron volts,” Snow explained. “But our paper is about cosmic ray particles of much higher energies, greater than 8 times 10 to the 18th electron volts. We can only speculate what the sources of these particles may be.”

Snow said physicists can learn the most about specific sources by studying the arrival directions of the very highest energy particles, since their measured arrival directions essentially point straight back to their sources.

Snow said the source must come from some faraway “extragalactic astrophysical catastrophes, something like two galaxies colliding or a galaxy with a super-massive black hole at its center which is exploding and shooting streams of ultra-high energy particles into intergalactic space.”

“We know there are galaxies which have an active galactic nucleus at their center — a very violent and hot center — which could be explained by one of these super-massive, exploding black holes,” he said.

“But,” Snow continued, “these highest energy, undeflected cosmic ray particles come to us very rarely which is one reason why the Pierre Auger Observatory plans to collect data for at least 10 more years, with planned upgrades to the current detectors, to observe more of the very highest energy particles whose arrival directions should point straight back to the objects that accelerated them to such energies.”

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