Where Do High-Energy Cosmic Rays Come From? A Star’s Last Gasp

Researchers compiled data from several telescopes to show that explosive stellar death can generate some of the fastest particles in the universe.

The Large Hadron Collider at CERN is one of the most ambitious undertakings in particle physics. For nearly $5 billion, scientists were able to build a ring of superconducting magnets chilled to temperatures colder than space that they can use to accelerate subatomic particles to speeds nearing that of light itself.

But nature does the job even better. For over a century, physicists have been flabbergasted by the existence of cosmic rays, which are charged particles—mostly protons—from outer space that bombard the Earth, thousands per square meter every second. Cosmic rays can reach our planet with speeds driven by over a peta-electron volt, or PeV, of energy. (That’s a quadrillion electron volts—a hundred times higher than what can be achieved with the LHC.) And though there’s no shortage of cosmic rays to study, scientists have mostly been in the dark about exactly what can push particles to such extreme speeds.

Earlier this month, a new paper in Physical Review Letters shed some light on this mystery. By combining data from NASA’s Fermi Gamma-ray Space Telescope with observations from nine other experiments, a team of five scientists has conclusively identified a supernova remnant as a source of PeV protons. Discovering these cosmic ray “factories”—called PeVatrons by the scientists who study them—will eventually help them characterize the environmental conditions that propel these particles and the role they play in the evolution of the cosmos.

“Identification of these PeVatrons will be a first step toward understanding the more energetic universe,” says University of Wisconsin-Madison astrophysicist Ke Fang, who led the discovery. So far, only a couple of potential PeVatrons have been tracked down in the Milky Way: the supermassive black hole at our galactic center, and a star-forming region that resides on the outskirts. In theory, supernova remnants—the gas and dust left by the explosive deaths of stars—should also be able to generate PeV protons, Fang says. But until now, there was no observational evidence to back that up.

“When massive stars explode, they produce these shock waves that propagate into the interstellar medium,” says Matthew Kerr, a physicist at the US Naval Research Laboratory and coauthor of the study. It’s theorized that protons get trapped in the magnetic field of supernova remnants, cycling around in the vicinity of the shock waves and getting boosted with each lap—“almost like surfing,” Kerr says—until they gain enough energy to escape. “But we can’t actually go there and put a particle detector in the supernova remnant to figure out if that’s true or not,” he says.

And though plenty of PeV protons fall to Earth, scientists have no way to tell which direction—much less what source—these particles come from. That’s because cosmic rays zigzag through the universe, bouncing off matter like ping-pong balls and gyrating through magnetic fields, making it impossible to trace them back to their origins. But with this supernova remnant, scientists noticed the bright glow of gamma rays that, unlike charged particles, travel in straight lines from their birthplace to Earth. That was a clue: If PeV protons were present, they might be interacting with the interstellar gas and producing unstable particles called pions, which quickly decay into gamma rays—the highest energy light there is, with wavelengths too small to be seen by the human eye.

Gamma rays from this supernova remnant have been seen by telescopes since 2007, but exceptionally energetic light wasn’t detected until 2020, when it was picked up by the HAWC Observatory in Mexico, piquing the interest of scientists hunting for galactic PeVatrons. When gamma rays reach our atmosphere, they can produce showers of charged particles that can be measured by telescopes on the ground. With data from HAWC, scientists were able to work backward and determine that these showers came from gamma rays emanating from the supernova remnant. But they were unable to say whether the light was generated by protons or speedy electrons—which can also radiate gamma rays, as well as lower-energy x-rays and radio waves.

To prove that PeV protons were the culprits, Fang’s research team compiled data across a broad range of energies and wavelengths that had been collected by 10 different observatories over the past decade. Then they turned to computer simulations. By tweaking different values, like the strength of the magnetic field or the density of the gas cloud, the researchers tried to reproduce the conditions necessary to account for all the different wavelengths of light they had observed. No matter what they adjusted, electrons couldn’t be the only source. Their simulations would only match the highest energy data if they included PeV protons as an additional source of light.

“We were able to exclude that this emission is dominantly produced by electrons because the spectrum we got out just wouldn’t match the observations,” says Henrike Fleischhack, an astronomer at the Catholic University of America who had first attempted this analysis two years ago with just the HAWC data set. Doing a multiwavelength analysis was key, Fleischhack says, because it allowed them to show, for example, that increasing the number of electrons at one wavelength led to a mismatch between data and simulation at another wavelength—meaning the only way to explain the full spectrum of light was with the presence of PeV protons.

“The result required a very careful attention to the energy budget,” says David Saltzberg, an astrophysicist at the University of California Los Angeles who was not involved in the work. “What this really shows is that you need many experiments, and many observatories, to answer the big questions.”

Looking ahead, Fang is hopeful that more supernova remnant PeVatrons will be found, which will help them figure out if this discovery is unique, or if all stellar corpses have the ability to accelerate particles to such speeds. “This could be the tip of the iceberg,” she says. Up-and-coming instruments like the Cherenkov Telescope Array, a gamma-ray observatory with over 100 telescopes being erected in Chile and Spain, may even be able to locate PeVatrons beyond our own galaxy.

Saltzberg also believes that next-generation experiments should be able to see neutrinos (tiny, neutral particles that can also result when pions decay) arriving from supernova remnants. Detecting these with the IceCube Neutrino Observatory, which hunts for their traces at the South Pole, would be even more of a smoking gun proving that these sites are PeVatrons because it would indicate the presence of pions. And Fang agrees: “It’ll be fantastic if telescopes like IceCube can see neutrinos directly from the sources because neutrinos are clean probes of proton interactions—they cannot be made by electrons.”

Ultimately, finding the PeVatrons of our universe is crucial for gleaning just how the relics of stellar death pave the way for new stars to be born—and how the highest-energy particles help fuel this cosmic cycle. Cosmic rays influence pressure and temperature, drive galactic winds, and ionize molecules in star-fertile regions like supernova remnants. Some of those stars may go on to form their own planets or one day explode into supernovas themselves, commencing the process all over again.

“Studying cosmic rays is almost as important to understanding the origins of life as studying exoplanets, or anything else,” Kerr says. “It’s all an energetic system that’s very complicated. And we’re just now coming to understand it.”

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