In mid-July 2026, an article was published in The Astrophysical Journal that brings us closer to understanding how the most energetic cosmic rays are born in the Milky Way. It concerns the object LHAASO J1912+1014u — one of the candidates for so-called PeVatrons, capable of accelerating particles to petaelectronvolt energies.
It all began with data from the Chinese LHAASO observatory, located on a high-altitude plateau. Its detectors recorded gamma-ray emission with energies above 100 TeV from an extended source that coincides in coordinates with the known TeV object HESS J1912+101. Such ultra-high-energy gamma rays usually appear when accelerated cosmic rays collide with atoms of interstellar gas. But the main question remained: are they protons or electrons? Leptonic (electron acceleration) and hadronic (proton acceleration) scenarios produce similar gamma-ray spectra at very high energies but require different conditions.
To figure this out, a group of astronomers led by Tsunefumi Mizuno from Hiroshima University delved into the archives of Fermi-LAT, NASA's orbital gamma-ray telescope. They analyzed 15 years of observations in the energy range from 400 MeV to hundreds of GeV. The standard model of galactic diffuse background left significant residuals in the data, so the scientists created an improved version. As a result, a clear excess of radiation with a hard spectrum (index around 2.1) emerged in the vicinity of the source. It spatially coincided with the position of the LHAASO/H.E.S.S. source.
The decisive step was the incorporation of high-precision FUGIN data — a survey of molecular clouds in the carbon monoxide line. The researchers constructed gas maps for two probable velocity components (around 25 and 60 km/s). These patterns perfectly described the GeV excess, confirming that the radiation originates precisely from the collisions of protons with dense clouds.
When the team constructed the overall spectrum from GeV to TeV, the leptonic scenario (electron acceleration) required overly stringent conditions and poorly agreed with the upper limits on X-ray emission. The hadronic scenario — proton acceleration — fit the data naturally. The proton spectrum is obtained with an index of ≈2.2, and the total energy injected into them is estimated at (1–5)×10⁴⁹ erg, depending on the distance. Near the source is an old pulsar, PSR J1913+1011, which likely supplies the system with energy.
This work is a good example of how patient data accumulation and joint analysis of different instruments allow us to gradually uncover the mechanisms that have been operating in our Galaxy for millions of years. LHAASO continues to find new candidates for PeVatrons, and similar studies show that many of them are most likely operating on protons. We are getting closer to understanding where and how nature accelerates particles to energies inaccessible even to the most powerful accelerators on Earth.
