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The most energetic cosmic rays pelting Earth are coming from outside our galaxy

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Finally answering a decades-old question

A rendering of cosmic rays showering particles onto Earth.
Image by A. Chantelauze, S. Staffi, and L. Bret

Astronomers have finally solved a long-standing mystery about the origins of cosmic rays, the highly energetic particles that zoom throughout space. For half a century, scientists haven’t been able to pin down where the most energetic rays in our Universe come from. But thanks to more than a decade of detecting cosmic rays from South America, astronomers have confirmed that these super energetic particles are coming from outside our galaxy.

Space is filled with cosmic rays — tiny fragments of atoms — all with varying amounts of energies. Many of the low- or medium-energy ones are thought to originate from within our galaxy, likely from supernovae, or exploding stars, which hurl high-speed particles out into space when they die. Then there are what are considered ultra high-energy cosmic rays: particles with energies millions of times greater than any particle ever observed on Earth. These types of rays are puzzling, mostly because no one is quite sure what is causing the particles to get so energetic. “We don’t know of a mechanism that can accelerate particles up to the energies we observe,” Greg Snow, a professor of physics at the University of Nebraska-Lincoln and one of the collaborators on this research, tells The Verge.

Now, astronomers are significantly closer to getting an answer, thanks to new research detailed today in Science. International astronomers spent 12 years detecting cosmic rays at the Pierre Auger Observatory, a facility in Argentina specifically designed to pick up these particles when they reach Earth and pelt our atmosphere. After observing more than 30,000 of the most energetic particles, the researchers created a map of their distributions in the sky. Sure enough, they found that most of these particles seemed to come from a part of the sky away from the center of the Milky Way Galaxy. “This is solid evidence that they are not coming from our galaxy,” David Nitz, a professor of physics at Michigan Tech University and another researcher on the study, tells The Verge.

It still doesn’t explain what is producing these particles, but the research does point scientists in the direction they need to look. The patch of sky that these rays seem to be coming from is known to have a large clustering of galaxies. It’s still unclear exactly which galaxies may be sending these energetic particles our way, but now researchers can start learning more about this general region of the Universe. “I predict there will be a flurry of papers now that this result has been confirmed where people try to correlate where the rays are coming from,” says Thomas Gaisser, a physics professor and cosmic ray researcher at the University of Delaware, who was not involved with the study.

One of the detectors at the Pierre Auger Observatory.
Image: The Pierre Auger Obseravtory

Figuring out the origins of cosmic rays is exactly what the Pierre Auger Observatory was built to do. The facility is home to 1,600 cosmic ray detectors that span an area of 1,200 square miles, all looking for traces of these particles when they mingle with our atmosphere. When the rays reach Earth, they slam into the gas molecules surrounding our planet, creating a whole bunch of secondary particles that then also collided with the molecules in our atmosphere. The results are called “air showers.” They’re made up of billions of little particles traveling at the speed of light that “rain” down on the Earth’s surface.

To measure these showers, the observatory uses specialized water tanks instead of telescopes. Whenever the air showers reach the ground, they pass through the water in the detectors, creating electromagnetic shockwaves that produce a strange blue glow. This phenomena is then picked up by light-detecting tubes mounted on the tanks. By using this technique, the observatory can pick up signs of the most energetic particles, which hit the atmosphere with energies greater than quintillions of electronvolts — that’s 1 X 10^18. “If you have a particle that has an energy of that order, that’s a macroscopic amount of energy,” says Snow. “It could be compared to a professional tennis player serving a tennis ball at 100 miles per hour. That’s a lot of energy.”

A rendering of how the Pierre Auger detectors measure light produced by air showers.
Image: The Pierre Auger Observatory

Finding the origins of these particles took quite a while, since the highest energy rays don’t hit Earth very often. Just once a year, an area a little less than a square mile will get hit by such a particle. But after 12 years of observations, the researchers were able to plot the general distributions of these rays, finding they weren’t evenly distributed throughout the sky, but seemed to originate from one general direction.

Still, there are a lot of unknowns about these rays. For one, the researchers only have a general direction from which they originate, but even that has some uncertainty. When cosmic rays enter the Milky Way, they have to pass through our galaxy’s magnetic field, which bends their direction slightly. And it’s not clear how much their directions change when that happens. “Because of this unknown amount of bending ... we can’t say they’re coming from any one cluster of galaxies,” says Nitz.

Finally knowing that these super energetic rays don’t start as Milky Way residents is the first major step in figuring out what these particles are. “It’s something that people have long thought to be the case, but there was no proof of it,” says Gaisser. “It’s nice to have a real result that shows these particles are coming from outside our galaxy.”