Milky Way Source Found for Extremely High-Energy Particles

An international team led by Hiroshima University has identified what appears to be the Milky Way’s highest-energy “proton PeVatron,” pinpointing the cosmic accelerator behind a long-sought class of ultra-energetic particles. The target, LHAASO J1912+1014u, is confirmed through a combined, multi-instrument analysis rather than inference from a single observation.

Cosmic rays are mostly protons (with a smaller fraction of electrons) that traverse interstellar space at energies far beyond what human-made accelerators typically achieve. The extreme upper end of galactic cosmic-ray energies reaches or exceeds 10^15 electron volts—known as a peta-electron-volt (PeV). Finding a source capable of accelerating protons above this threshold is considered one of the most compelling problems in modern astrophysics.

The source was initially flagged by the Tibet AS gamma experiment and later by China’s Large High Altitude Air Shower Observatory (LHAASO). These observatories detected very high-energy gamma rays above 0.1 PeV, including emissions associated with LHAASO J1912+1014u. Because gamma rays often carry only about one-tenth the energy of their parent cosmic rays, such detections made the object a strong candidate for PeV proton acceleration.

However, proving a proton PeVatron is difficult. At PeV scales, cosmic-ray electrons can also generate gamma rays in related energy bands, and the angular resolution of current gamma-ray instruments can limit definitive separation between competing models. The breakthrough came by expanding the dataset beyond gamma rays alone.

Researchers incorporated observations from the Fermi Large Area Telescope (Fermi-LAT), the Nobeyama 45-m radio telescope in the FUGIN survey, and the Chandra X-ray Observatory. Together, these facilities cover a wide range of photon energies—from radio wavelengths through GeV and TeV gamma rays to X-rays—enabling detailed multiwavelength modeling of the physical processes at work.

The gamma-ray spectrum from LHAASO J1912+1014u shows a smooth extension from above 100 trillion electron volts down to about 400 million electron volts. This continuity makes an electron-acceleration-only explanation unlikely on energetic grounds. In addition, the GeV gamma-ray spatial pattern matches the distribution of interstellar gas traced by FUGIN radio data, reinforcing a hadronic scenario in which protons interact with matter to produce gamma rays.

Finally, Chandra data indicate weak diffuse X-ray emission, further narrowing the viable interpretations. By bundling “three arrows”—GeV gamma-ray measurements, radio-derived gas tracers, and X-ray constraints—the team argues that the most consistent explanation is that LHAASO J1912+1014u is actively accelerating protons into the PeV range.

These results were published in The Astrophysical Journal on July 16, 2026, and the authors note that dozens of PeVatron candidates remain in the Milky Way. Future work will apply similar multi-instrument strategies to systematically test other potential accelerators.

Keywords

Cosmic rays; PeVatron; LHAASO; Fermi-LAT; FUGIN; Chandra; gamma rays; hadronic interactions; interstellar gas; multiwavelength modeling