Researchers have compiled data from several telescopes showing that explosive star deaths can produce some of the fastest particles in the universe.
The European Organization for Nuclear Research of CERN’s Large Hadron Collider is one of the most ambitious undertakings in particle physics. Scientists have spent nearly $5 billion to build a ring of superconducting magnets, which are colder than space, and which they can use to accelerate subatomic particle to near the speed of light itself.
But nature does an even better job. For more than a century, physicists have been shocked by the existence of cosmic rays, charged particles from outer space-mostly protons-that bombard the earth with thonds of them per second per square metre. Cosmic rays can reach our planet at speeds driven by more than one peta-electron volt, or PEV, of energy. That’s 400 million electron volts, a hundred times faster than a Large Hadron Collider can reach. Although there is no shortage of research on cosmic rays, most scientists know nothing about exactly what drives particles to such extreme speeds.
A new paper in the Physical Review Letters earlier this month shed some light on the mystery. By combining data from the National Aeronautics and Space Administration Fermi Gamma-ray Space Telescope with observations from nine other experiments, a team of five scientists has finally identified a supernova remnant as the source of PEV protons. The discovery of these cosmic ray“Factories”-called Pevatrons by the scientists who study them-will eventually help them determine the environmental conditions driving these particles and their role in the evolution of the universe.
“Identifying these PeVatrons would be a first step to understanding a more energetic universe,” said Ke Fang, a University of Wisconsin–Madison astrophysicist who led the discovery. So far, only a few potential PeVatrons have been tracked in the galaxy — the supermassive black hole of our Galactic Center and a star-forming region that lives in the periphery. In theory, supernova remnant — gas and dust left behind by the star’s explosive death — should also be able to produce PeV protons, Fang said. But until now, there has been no observational evidence to support this.
“When massive stars explode, they create these shock waves that propagate into the interstellar medium,” said Matthew Quer, a physicist at the U.S. Naval Research Laboratory and a co-author of the study. Presumably, the protons are trapped in the supernova remnant’s magnetic field, looping around the shock wave, getting a boost with each turn — “Almost like surfing,” Quer said — until they gain enough energy to escape. “But we can’t really go there and put the particle detector in the supernova remnant to see if that’s true,” he says.
Although large numbers of PEV protons have landed on Earth, scientists have no way of knowing which direction the particles came from, let alone where they came from. This is because cosmic rays zigzag across the universe, bouncing off matter like ping-pong balls and swirling around in magnetic fields, making it impossible to trace their origin. But in the remnants of the supernova, scientists noticed that gamma rays, unlike charged particles, travel in a straight line from their birthplace to Earth. Here’s a clue: if PEV protons exist, they may interact with interstellar gas and produce unstable particles called protons, which rapidly decay into gamma rays, the most energetic light, its wavelength is too small for the human eye to see.
Gamma rays from the supernova remnant have been seen by telescopes since 2007, but it wasn’t until 2020 that the unusual high-energy light was spotted by the HAWC Observatory in Mexico, the search for the Milky Way’s PeVatrons piqued the interest of scientists. When gamma rays reach our atmosphere, they can produce a shower of charged particles that can be measured by telescopes on the ground. Using HAWC’s data, scientists were able to extrapolate back and determine that the showers came from gamma rays emitted by supernova remnants. But they can’t say whether the light is produced by protons or high-speed electrons-they can also emit gamma rays, as well as low-energy X-rays and radio waves.
To prove that PEV protons are the culprit, Fang’s team compiled data on a wide range of energies and wavelengths collected by 10 different observatories over the past decade. Then they turned to computer simulations. By adjusting for different values, such as the strength of a magnetic field or the density of a gas cloud, the researchers tried to recreate the necessary conditions for all the different wavelengths of light they observed. No matter how they adjust, electrons can’t be the only source. Their simulation matches the highest-energy data only if it includes PEV protons as an additional light source.
Astronomer Henrike Fleischhack, of the Catholic University of the United States, said: “We can rule out the possibility that this emission is mainly caused by electrons, because the spectra we get do not match the observations. Fleischhake says that performing multi-wavelength analysis is key because it allows them to show, for example, increasing the number of electrons at one wavelength leads to mismatches between the data and the simulations at the other-meaning that the only way to explain the full spectrum is to have PEV protons.
“This result requires a very careful look at the energy budget,” said David Saltzberg, an astrophysicist at the University of California, Los Angeles, who was not involved in the work. “What it really shows is that you need a lot of experiments and a lot of observatories to answer the big questions.”
Looking ahead, Fang hopes to discover more supernova remnant PeVatrons, which will help them figure out if this discovery is unique, or whether all stellar corpses have the ability to accelerate particles to that speed. “This could be the tip of the iceberg,” she said. Emerging instruments like the Pavel Cherenkov Telescope Array, a 100-telescope gamma-ray observatory in Chile and Spain, may even be able to find PeVatrons beyond our own Milky Way galaxy.
Salţberg also thinks the next generation of experiments should be able to see neutrinos (tiny neutral particles that can be produced when ions decay) coming from supernova remnants. Detecting these neutrinos with the icecube Neutrino Observatory, which hunts them down at the South Pole, would be more of a smokescreen to prove that the sites are PeVatrons, because it would indicate the presence of ions. Fang agrees. “It would be wonderful if a telescope like icecube could see neutrinos directly at their source, because neutrinos are clean probes of proton interactions-they can’t be produced by electrons.”
Ultimately, finding Pevatrons in our universe is critical to understanding how the remnants of star death pave the way for new stars to be born — and how the highest-energy particles help drive this cosmic cycle. Cosmic rays affect pressure and temperature, driving winds in the Milky Way and ionizing molecules in star-rich regions such as supernova remnant. Some of these stars may go on to form their own planets, or one day explode into supernovae and start 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 said. “It’s a very complex energy system. And we’re just beginning to understand it.”
