Observation by NASA’s Fermi Gamma-ray Space Telescope identified GeV gamma-ray excess toward LHAASO J1912-1014u and confirm it to be a proton PeVatron through multiwavelength observations and modeling. The source is marked by a solid circle and is largely extended, with a diameter of more than 1 degree. For comparison, the size of the Moon is indicated by a dashed circle.
Image Credit: Adapted from Tsunefumi Mizuno, et al. Astrophysical Journal. July 16, 2026
Scientific Frontline: Extended "At a Glance" Summary: LHAASO J1912+1014u
The Core Concept: LHAASO J1912+1014u has been identified as a proton "PeVatron," an elusive celestial object capable of accelerating protons to energies reaching or exceeding one quadrillion (10^15) electron volts (PeV).
Key Distinction/Mechanism: Unlike electron accelerators, which are ruled out by the smooth, wide-range gamma-ray emission spectrum, this source demonstrates a hadronic origin supported by the correlation between gamma-ray data and interstellar gas distribution.
Major Frameworks/Components:
- Fermi Large Area Telescope (Fermi-LAT) gamma-ray data (GeV range).
- FUGIN radio telescope survey data tracing interstellar gas distribution.
- Chandra X-ray Observatory data confirming weak diffuse X-ray emissions.
- Large High Altitude Air Shower Observatory (LHAASO) and Tibet AS gamma experiment data (TeV range).
Branch of Science: Astrophysics, High-Energy Particle Physics, and Observational Cosmology.
Future Application: Comprehensive mapping and characterization of other candidate PeVatrons in the Milky Way to better understand the sources of galactic cosmic rays.
Why It Matters: Identifying proton PeVatrons is critical for understanding the origin of galactic cosmic rays and the high-energy processes that shape the evolution of the Milky Way galaxy.
Cosmic rays are composed primarily of protons, along with a small number of electrons, and can reach energies even higher than those produced by human-made accelerators. Given that human-made accelerators, such as the Large Hadron Collider on the Swiss-French border, can accelerate protons to near the speed of light, it is not surprising that these highly energetic particles can influence cosmic events across the galaxy.
An international team of researchers, led by Hiroshima University, has conclusively identified an accelerator of the highest-energy cosmic-ray protons in our galaxy by analyzing data from three major terrestrial and space-based observatories. This discovery could help scientists better understand the nature of these fast-moving particles, which fill interstellar space and influence cosmic events throughout the Milky Way.
"This immense energy makes cosmic rays important in astronomy and astrophysics," said Tsunefumi Mizuno, the study's first and corresponding author and an 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 one quadrillion ($10^{15}$) electron volts, or one petaelectronvolt (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 have identified one such object, previously known as LHAASO J1912+1014u."
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 represent the most energetic form of electromagnetic radiation and can originate from cosmic-ray sources, possess energies approximately one-tenth those of their parent cosmic-ray particles. Consequently, these sub-PeV gamma-ray sources, including LHAASO J1912+1014u, are potential cosmic-ray PeVatron candidates. Previous studies in the field have proposed that the source might be a pulsar wind nebula or other debris from a massive stellar explosion.
"However, data from the Tibet AS-gamma and LHAASO experiments alone cannot clearly identify proton PeVatrons because PeV cosmic-ray electrons can also produce lower-energy gamma rays," Mizuno said, explaining that the experiments’ spatial resolution is limited, preventing researchers from delving deeply enough to confirm a proton PeVatron.
However, additional data had been collected by other experiments: the Fermi Large Area Telescope (Fermi-LAT), led by NASA, to which Hiroshima University contributed instrumentation development and operational support; 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.
Discovered in 2024, LHAASO J1912+1014u is located within the constellation Aquila, close to Altair, a prominent star in the Summer Triangle. It was originally considered a supernova remnant until emissions above 100 teraelectronvolts (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.
Fermi-LAT recorded gamma-ray data with energies around one gigaelectronvolt (GeV), or one billion electron volts, while Chandra provided data at lower energies, and FUGIN recorded data at even lower energies. By combining these data with teraelectronvolt (TeV) observations from instruments such as LHAASO, the researchers constructed a detailed profile of LHAASO J1912+1014u as a proton PeVatron, allowing them to rule out alternative scenarios.
First, the gamma-ray emission smoothly extends from over 100 trillion electron volts down to 400 million electron volts, making the hypothesis that LHAASO J1912+1014u is an electron accelerator unlikely based on energetic arguments, according to Mizuno. Second, the GeV gamma-ray map corresponds closely to the distribution of interstellar gas traced by FUGIN radio data, strongly supporting the proton PeVatron scenario. Third, Chandra X-ray data revealed that the diffuse X-ray emission is very weak, further reinforcing this conclusion.
"This research was achieved through 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 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 also characterized the properties of the accelerated particles. This is crucial for understanding the nature of the source. According to Mizuno, there are dozens of cosmic-ray proton PeVatron candidates in the Milky Way. The researchers plan to comprehensively examine other potential PeVatron sources next.
Funding: This work was performed in part under U.S. 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) and the Japan Society for the Promotion of Science (JSPS) KAKENHI (23K25882, 23H04895, 22H00152, 24H00246, and 24K17093).
Published in journal: The Astrophysical Journal
Authors: Tsunefumi Mizuno, Hidetoshi Sano, Takeru Murase, Tomohiko Oka, Hiromasa Suzuki, and Naohito Nakahara
Source/Credit: Hiroshima University
Edited by: Scientific Frontline
Reference Number: asph071726_01
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