A source of extremely high-energy particles in the Milky Way identified

Cosmic rays are made primarily of protons with a few electrons sprinkled in, and they can reach energies even higher than what human-made accelerators can produce. Considering human-made accelerators, such as the Large Hadron Collider on the border of Switzerland and France, can move protons to near the speed of light, it’s no wonder that these super-energetic particles can influence cosmic events across the galaxy.

An accelerator of the highest-energy cosmic-ray protons in our galaxy has been identified conclusively, thanks to a Hiroshima University-led international team of researchers assessing data from three major observatories on Earth and in space. This could help scientists better understand the nature of these fast-moving particles that fill the space between stars and influence cosmic events across the Milky Way galaxy.

The findings were published in The Astrophysical Journal.

“This immense energy makes cosmic rays important in astronomy and astrophysics,” said first and corresponding author of the study, Tsunefumi Mizuno, associate professor at Hiroshima University’s Hiroshima Astrophysical Science Center. He explained that these energies are measured in electron volts—the energy an electron gains when it moves from a resting state and increases its electrical potential by one volt.

“The highest energy of galactic cosmic rays can reach and exceed 1 quadrillion (10¹⁵) electron volts, or a peta electron volt (PeV). Finding a cosmic-ray proton accelerator above that PeV level, called a proton PeVatron, is one of the most exciting topics in modern astrophysics, and we identified one such object previously known as LHAASO J1912+1014u.”

Why this source stood out

The Tibet AS gamma experiment, a project led by Japan and China since 1990, and later China’s Large High Altitude Air Shower Observatory (LHAASO) found dozens of gamma-ray sources above 0.1 PeV, including one named LHAASO J1912+1014u.

These gamma-rays, which are the most energetic electromagnetic radiation and can originate from cosmic-ray sources, have energies about one-tenth of their parent cosmic-ray particles.

Accordingly, these sub-PeV gamma-ray sources, including the one named LHAASO J1912+1014u, are potential cosmic ray PeVatron candidates. Previous studies in the field propose that the source might be a pulsar wind nebula or other debris from a massive star explosion.

“However, data from Tibet AS gamma and LHAASO experiments alone cannot clearly identify proton PeVatrons because PeV cosmic ray electrons can also produce the lower energy gamma-rays,” Mizuno said, explaining that the experiments’ image resolution is limited, so researchers cannot delve deeply enough to confirm a proton PeVatron.

Three observatories sharpened the picture

But data had been collected by other experiments: Fermi Large Area Telescope (Fermi-LAT), led by NASA and to which Hiroshima University contributed to the instrumentation development and operation; the FOREST Unbiased Galactic plane Imaging survey with the Nobeyama 45-m telescope (FUGIN), led by Japan; and the Chandra X-ray Observatory, led by NASA.

LHAASO J1912+1014u was discovered in 2024, and is located within the constellation Aquila, and close to Altair, a famous star constituting the Summer Triangle. It was originally considered a supernova remnant, until emissions above 100 TeV were detected.

“With data from multiple experiments, we have studied LHAASO J1912+1014u in detail,” Mizuno said. He noted that the experiments provide data across a wide wavelength range from radio to gamma-rays, enabling this broad investigation with comprehensive multiwavelength modeling.

The gamma-ray data from Fermi-LAT clocked in with energies around a giga electron volt (GeV), or one billion electron volts; while Chandra provided data on lower energies and FUGIN with still lower energies. By combining this data with information in a tera electron volt (TeV) from instruments including LHAASO, the researchers could paint a detailed picture of LHAASO J1912+1014u as a proton PeVatron and rule out other possible scenarios.

Evidence points to proton acceleration

First, the gamma-ray emission smoothly extends from over 100 trillion electron volts down to 400 million electron volts, making the possible explanation that LHAASO J1912+1014u is an electron accelerator unlikely based on energy arguments, according to Mizuno.

Second, the GeV gamma-ray map matches well with the distribution of interstellar gas traced by FUGIN radio data, which strongly supports the proton PeVatron scenario. Third, Chandra X-ray data revealed that diffuse X-ray emission is very weak, further reinforcing the scenario.

“This research is achieved by a team effort. There is an old Japanese saying, “One arrow is easy to break, but three arrows bundled together are not,'” Mizuno said. “In this study, three arrows—Fermi-LAT GeV gamma-ray data, FUGIN radio data and Chandra X-ray data—are bundled together through a detailed multiwavelength modeling, revealing that our target, LHAASO J1912+1014u, is a cosmic-ray proton PeVatron.”

Mizuno also emphasized that their study not only identified a proton PeVatron, but characterized properties of the accelerated particles. This is crucial to understand the nature of the source. According to him, there are dozens of cosmic-ray proton PeVatron candidates in the Milky Way. The researchers plan to comprehensively examine other potential PeVatron sources next.

Who’s behind this story?

Sadie Harley

BSc Life Sciences & Ecology. Microbiology lab background with pharmaceutical news experience in oil, gas, and renewable industries.

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Andrew Zinin

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