Cosmic rays consist mostly of protons, along with a much smaller number of electrons. Some reach energies greater than those produced by human-built particle accelerators. The Large Hadron Collider on the border of Switzerland and France can propel protons to nearly the speed of light, yet the natural accelerators found in space can push particles to even more extreme energy levels.
An international research team led by Hiroshima University confirmed the galactic accelerator after combining observations from three major observatories operating on Earth and in space. The results were published in The Astrophysical Journal on July 16, 2026.
A Natural Accelerator With Extraordinary Power
"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.
Cosmic ray energies are measured in electron volts, a unit describing the energy an electron gains when its electrical potential increases by one volt.
"The highest energy of galactic cosmic rays can reach and exceed one 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."
A proton PeVatron is a natural cosmic accelerator capable of pushing protons beyond the PeV threshold. Identifying these objects is difficult because scientists must distinguish the signals produced by protons from those created by highly energetic electrons.
Gamma Rays Pointed to a Possible PeVatron
The Tibet AS gamma experiment, which has been led by Japan and China since 1990, and China's Large High Altitude Air Shower Observatory (LHAASO) have detected dozens of gamma-ray sources with energies above 0.1 PeV. One of those sources is LHAASO J1912+1014u.
Gamma rays are the most energetic form of electromagnetic radiation. They can be produced when cosmic-ray particles interact with their surroundings, and their energies are typically about one-tenth of the energy carried by the cosmic rays that created them.
For that reason, sources producing gamma rays below the PeV range can be considered possible PeVatron candidates. Earlier research suggested that LHAASO J1912+1014u could be a pulsar wind nebula or another type of debris left behind by the explosion of a massive star.
"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.
The limited image resolution of those observations meant researchers could not examine the source closely enough to determine whether protons or electrons were responsible for the gamma rays.
Three Observatories Provided the Missing Evidence
Additional information was available from several other instruments. These included the Fermi Large Area Telescope (Fermi-LAT), a NASA-led mission that Hiroshima University helped develop and operate; the FOREST Unbiased Galactic plane Imaging survey with the Nobeyama 45-m telescope (FUGIN), led by Japan; and NASA's Chandra X-ray Observatory.
LHAASO J1912+1014u was discovered in 2024. It is located in the constellation Aquila near Altair, one of the well-known stars that forms the Summer Triangle. Scientists initially classified the object as a supernova remnant, but that interpretation became less certain after emissions above 100 TeV were detected.
"With data from multiple experiments, we have studied LHAASO J1912+1014u in detail," Mizuno said.
Together, the observatories supplied measurements across a broad portion of the electromagnetic spectrum, ranging from radio waves to gamma rays. This allowed the researchers to build a detailed multiwavelength model of the source.
Combining Signals Across the Energy Spectrum
Fermi-LAT measured gamma rays with energies near a giga-electron-volt (GeV), equal to one billion electron volts. Chandra collected X-ray observations at lower energies, while FUGIN supplied radio data at still lower energies.
The team combined these measurements with tera-electron-volt (TeV) observations from instruments including LHAASO. The resulting picture strongly indicated that LHAASO J1912+1014u is a proton PeVatron and allowed the researchers to eliminate other possible explanations.
Three main findings supported that conclusion.
First, the gamma-ray signal extended smoothly from more than 100 trillion electron volts down to 400 million electron volts. According to Mizuno, the wide energy range made it unlikely that the source was primarily accelerating electrons.
Second, the distribution of GeV gamma rays closely matched the pattern of interstellar gas mapped through FUGIN radio observations. That connection is expected when high-energy protons collide with surrounding gas and produce gamma rays.
Third, Chandra detected only very weak diffuse X-ray emission. A source dominated by accelerated electrons would generally be expected to produce stronger X-ray signals, so the faint emission further supported the proton explanation.
Three Sets of Data Reveal One Cosmic Engine
"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. "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."
The study did more than identify the source as a PeVatron. The researchers also examined the characteristics of the particles being accelerated, information that may help reveal what kind of object is powering the process.
Mizuno said there are dozens of other possible proton PeVatrons in the Milky Way. The team now plans to study those candidates more comprehensively to determine how many can be confirmed and what types of cosmic objects produce them.
Naoto Nakahara at Hiroshima University; Hidetoshi Sano & Takeru Murase at Gifu University; Tomohiko Oka at Julius-Maximilians-Universität Würzburg; and Hiromasa Suzuki at Miyazaki University co-authored the study.
The Fermi LAT Collaboration acknowledges generous ongoing support from the National Aeronautics and Space Administration (NASA) and the Department of Energy (DOE) in the United States; the Commissariat à l'Energie Atomique and the Centre National de la Recherche Scientifique / Institut National de Physique Nucléaire et de Physique des Particules in France; the Agenzia Spaziale Italiana and the Istituto Nazionale di Fisica Nucleare in Italy; the Ministry of Education, Culture, Sports, Science and Technology (MEXT), High Energy Accelerator Research Organization (KEK) and Japan Aerospace Exploration Agency (JAXA) in Japan; and the K. A. Wallenberg Foundation, the Swedish Research Council and the Swedish National Space Board in Sweden. Additional support for science analysis from the Istituto Nazionale di Astrofisica in Italy; and the Centre National d'Études Spatiales in France is gratefully acknowledged.
This work was performed in part under US Department of Energy (DOE) Contract DE-AC02-76SF00515. This work was also supported in part by a University Research Support Grant from the National Astronomical Observatory of Japan (NAOJ); the Japan Society for the Promotion of Science (JSPS) KAKENHI (23K25882, 23H04895, 22H00152, 24H00246, 24K17093). %!s()
