Using an observatory on the edge of the Tibetan Plateau, astronomers have spotted the highest energy light ever, gamma ray photons up to 1.4 petaelectronvolts (PeV). They have traced these extreme photons back to a dozen of their likely sources: powerful factories in the Milky Way Galaxy that accelerate charged particles called cosmic rays. The results are challenging theorists’ understanding of what these factories are and how they generate such high-energy light.
“The findings are extremely important and impressive,” says Petra Huentemeyer, an astrophysicist at Michigan Technological University and spokesperson for a rival gamma ray telescope, the High-Altitude Water Cherenkov Observatory (HAWC) in Mexico. “It’s a giant leap toward finally understanding the origin of the highest energy cosmic rays.”
Discovered more than 100 years ago, cosmic rays are charged particles, including protons and other atomic nuclei, that have been accelerated nearly to the speed of light. Their sources are poorly understood because interstellar magnetic fields bend them on their path to Earth. However, as cosmic rays rocket away from their sources, they also emit photons, usually about one-tenth as energetic as the cosmic rays themselves, that follow a straight path to Earth. Although Earth’s atmosphere blocks this gamma ray light, when the photons slam into air molecules, they create showers of secondary particles and faint blue Cherenkov light that astronomers can look for.
China’s Large High Altitude Air Shower Observatory (LHAASO) aims to catch the air showers associated with the highest energy gamma rays, which in turn correspond to the highest energy cosmic rays. LHAASO is a cluster of detectors spanning more than 1 square kilometer on Haizi Mountain, 4410 meters above sea level in Sichuan province. More than 5000 detectors spread across the site capture particles associated with the highest energy strikes, while more than 1000 muon detectors, buried underground, help rule out particle showers associated with unrelated cosmic rays that constantly pepper Earth. Before LHAASO began operations in 2019, most detectors worked in much lower energy bands. But the new results show the universe is capable of far higher accelerations.
Using data from LHAASO’s first year of operation, Cao Zhen from the Institute of High Energy Physics of the Chinese Academy of Sciences and his colleagues detected more than 530 photons with energies greater than 0.1 PeV, they reported yesterday in Nature. The photons were traced to 12 cosmic ray factories capable of PeV accelerations—100 times more energetic than collisions at the world’s most powerful atom smasher, the Large Hadron Collider. The sources, which the team calls “PeVatrons,” include long-suspected accelerators, such as the Crab nebula, the site of an ancient supernova, the final explosion of a dying star, and home to a powerful pulsar, a dense neutron star. But the highest energy photons came from a surprising source: the Cygnus Cocoon, a stellar nursery 4600 light-years from the Sun. “PeVatrons are basically everywhere in our galaxy,” Cao says.
These observations will leave theorists scratching their heads to explain how PeVatrons actually work. For instance, models have predicted strong magnetic fields from the pulsar in the Crab nebula can boost particles to 0.1 PeV, but to reach 1 PeV, Cao says, all the parameters need to be pushed to the extreme.
At the Cygnus Cocoon, Huentemeyer says, the acceleration mechanism could be powerful shock waves generated by strong winds emanating from massive newborn stars. LHAASO’s observation is consistent with another, reported in March by astronomers using HAWC: 0.1-PeV gamma rays from the Cygnus constellation, she adds.
Star-forming regions should be considered “a serious alternative or addition to supernovae remnants” as PeVatron candidates, Felix Aharonian, an astrophysicist at the Dublin Institute for Advanced Studies and a co-author of the Nature paper, said at a press conference announcing the results. “In this PeVatron competition of ‘young stars versus dead stars,’” he says, “the score is one to zero in favor of young stars.”
And LHAASO hasn’t yet reached its full power. After the construction is completed sometime next month, Cao says, the facility will search for more PeVatrons, pushing into an even higher energy range.