An international research team led by Hiroshima University has confirmed that a high-energy celestial object known as LHAASO J1912+1014u functions as a cosmic-ray proton accelerator, or proton PeVatron, capable of pushing protons beyond one quadrillion electron volts. Published in The Astrophysical Journal on July 16, 2026, the discovery combines multiwavelength observations from space- and ground-based observatories to rule out electron-driven gamma-ray emissions.
Detecting a Natural Cosmic Accelerator in Aquila
Scientists measure cosmic ray energies in electron volts, describing the energy an electron gains when its electrical potential increases by one volt. According to Tsunefumi Mizuno, associate professor at Hiroshima University’s Hiroshima Astrophysical Science Center and first author of the study, galactic cosmic rays can reach and exceed one quadrillion 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,” Mizuno said.
Discovered in 2024, LHAASO J1912+1014u sits in the constellation Aquila near Altair, a prominent star in the Summer Triangle. Researchers initially classified the source as a supernova remnant. However, that interpretation lost certainty after instruments detected emissions exceeding 100 tera-electron-volts (TeV).
Did you know? A peta electron volt (PeV) is equal to one quadrillion (
) electron volts. Identifying natural proton PeVatrons requires scientists to separate proton signals from those generated by high-energy electrons.
Why Previous Gamma-Ray Observations Fell Short
Prior to the new study, the Tibet AS gamma experiment—led by Japan and China since 1990—along with China’s Large High Altitude Air Shower Observatory (LHAASO), detected dozens of gamma-ray sources with energies above 0.1 PeV. LHAASO J1912+1014u emerged among them as a prime candidate.
Earlier research suggested the object might be a pulsar wind nebula or debris left behind by a massive star’s explosion. Yet, data from the Tibet AS gamma and LHAASO facilities alone could not definitively confirm a proton PeVatron.
“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. Limited image resolution prevented researchers from examining the source closely enough to isolate whether protons or electrons drove the gamma-ray production.
Three Observatories Provide the Missing Evidence
To overcome resolution limits, the team incorporated data from NASA’s Fermi Large Area Telescope (Fermi-LAT)—developed and operated with contributions from Hiroshima University—alongside the FOREST Unbiased Galactic plane Imaging survey with the Nobeyama 45-m telescope (FUGIN), led by Japan, and NASA’s Chandra X-ray Observatory.
“With data from multiple experiments, we have studied LHAASO J1912+1014u in detail,” Mizuno said.
These instruments spanned radio waves to gamma rays, enabling a comprehensive multiwavelength model. Fermi-LAT measured gamma rays near a giga-electron-volt (GeV), Chandra gathered lower-energy X-rays, and FUGIN supplied radio data. Combining these with tera-electron-volt observations from LHAASO verified the proton PeVatron classification.
Three Pillars Supporting the Proton PeVatron Conclusion
- Smooth Gamma-Ray Spectrum: The signal extended continuously from over 100 trillion electron volts down to 400 million electron volts, making electron acceleration unlikely according to Mizuno.
- Spatial Gas Correlation: The distribution of GeV gamma rays closely matched interstellar gas patterns mapped via FUGIN radio observations, consistent with high-energy protons colliding with surrounding gas.
- Faint X-Ray Emissions: Chandra detected only very weak diffuse X-ray emission, whereas an electron-dominated source typically produces much stronger X-ray signals.
“This research is achieved by team effort. There is an old Japanese saying: ‘One arrow is easy to break, but three arrows bundled together are not,'” Mizuno said, describing how Fermi-LAT GeV gamma-ray data, FUGIN radio data, and Chandra X-ray data united to reveal the cosmic engine.
Mizuno noted that dozens of additional potential proton PeVatrons exist within the Milky Way. The research team plans to study these candidates comprehensively to establish how many can be confirmed and what types of cosmic objects generate them.
Frequently Asked Questions
What is a proton PeVatron?
A proton PeVatron is a natural cosmic accelerator capable of pushing protons beyond one peta electron volt (one quadrillion electron volts).
Where is LHAASO J1912+1014u located?
The object is located in the constellation Aquila near Altair, one of the stars forming the Summer Triangle.
Which observatories contributed data to confirm the accelerator?
Researchers combined observations from the Tibet AS gamma experiment, LHAASO, Fermi-LAT, FUGIN on the Nobeyama 45-m telescope, and NASA’s Chandra X-ray Observatory.
Who led the international research team?
The study was led by Hiroshima University, with findings published in The Astrophysical Journal on July 16, 2026.
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