Study: Cygnus X-3 Emits Petaelectronvolt Gamma Rays, Hinting at Galactic 'PeVatron'

An X-ray binary system about 24,000 light-years from Earth is behaving like a natural particle accelerator, hurling out some of the most energetic light ever seen from a known star system, according to a new study.

Researchers with China’s Large High Altitude Air Shower Observatory, or LHAASO, report that the well-studied source Cygnus X-3 is emitting gamma-ray photons reaching up to petaelectronvolt (PeV) energies. That is on par with, and in some cases beyond, what human-made accelerators such as the Large Hadron Collider can achieve for individual particles.

In a preprint posted Dec. 18, 2025, and updated most recently on April 12, 2026, the LHAASO Collaboration writes, “We report the discovery of variable γ-rays up to petaelectronvolt from Cygnus X-3.” The team says the gamma-ray signal was detected “with a statistical significance of approximately 10 σ,” a level physicists typically regard as decisive.

The paper, titled “Cygnus X-3: A variable petaelectronvolt gamma-ray source” and submitted to the journal National Science Review, has not yet completed peer review. Independent confirmation from other telescopes is also still pending.

Cygnus X-3 is an “X-ray binary,” a compact object — likely a black hole or neutron star — in a tight 4.8-hour orbit around a massive Wolf-Rayet star. The system lies roughly 7.4 kiloparsecs, or about 24,000 light-years, from Earth in the crowded Cygnus region of the Milky Way. It launches powerful jets and bright radio outbursts, which has led astronomers to categorize it as a “microquasar,” a smaller analog of the giant quasars found in distant galaxies.

LHAASO, a ground-based observatory in southwest China designed to catch very-high-energy gamma rays from about 100 gigaelectronvolts up to the PeV range, has played a central role in opening what some researchers call PeV gamma-ray astronomy. In 2021, it reported photons up to about 1.4 PeV from several sources in and around Cygnus, including an extended “cocoon” of ultra-high-energy emission.

The new analysis focuses on Cygnus X-3 itself rather than the larger region. According to the preprint’s abstract, the team reconstructs an intrinsic spectral energy distribution — a measure of energy output across gamma-ray energies — from 0.06 PeV (60 trillion electronvolts) up to 3.7 PeV. After correcting for the expected absorption of gamma rays by the cosmic microwave background, the spectrum shows a pronounced rise toward around 1 PeV.

LHAASO also reports that the PeV emission is not steady. The gamma-ray flux appears to vary on month-long timescales with a statistical significance of 8.6 sigma, indicating a strong case that the source brightened and dimmed during the observation period. A high state in the PeV band coincides with a high state of billion-electronvolt (GeV) gamma rays detected from Cygnus X-3 by NASA’s Fermi Large Area Telescope, suggesting related activity across a wide energy range.

The most intriguing but least certain timing result is a possible connection to the system’s 4.8-hour orbit. The preprint describes “3.2 σ evidence for orbital modulation” in the PeV gamma-ray signal — a hint that the emission may wax and wane as the compact object circles its stellar companion. Because 3.2 sigma falls well below the usual threshold for discovery, the authors classify this as evidence only, not a firm detection. Further data and analysis will be needed to verify whether PeV gamma rays reliably track the orbital period.

If the results hold, they would make Cygnus X-3 a confirmed “PeVatron,” an astrophysical accelerator that can boost particles to PeV energies. For decades, astronomers have debated the origin of the highest-energy cosmic rays in the Milky Way, especially those near a feature in the cosmic-ray spectrum known as the “knee,” around a few PeV. Supernova remnants, star-forming regions and compact binaries have all been proposed as sources.

The LHAASO team argues that Cygnus X-3’s PeV gamma rays and their variability “can be naturally explained” if they are produced in the innermost region of the system’s relativistic jet by photomeson, or pγ, processes. In that scenario, protons are accelerated to tens of PeV and collide with intense radiation fields near the compact object, producing short-lived particles that decay into gamma rays and neutrinos.

Reaching tens of PeV would put Cygnus X-3 among the most powerful known Galactic accelerators, strengthening the case that microquasars can contribute significantly to the population of high-energy cosmic rays.

Photomeson processes should also generate high-energy neutrinos — nearly massless particles that pass through most matter unimpeded. To date, no clear neutrino counterpart to Cygnus X-3 has been announced. Neutrino observatories such as IceCube at the South Pole have reported only low-significance hints from parts of the broader Cygnus region. If Cygnus X-3 is confirmed as a PeV gamma-ray source, it will likely become a priority target for IceCube, the Mediterranean-based KM3NeT and other neutrino experiments in coordinated searches.

The claim comes with important caveats. The Cygnus region is crowded with gamma-ray emitters, including the extended cocoon, supernova remnants and massive star clusters. Separating diffuse emission from compact sources can be difficult, and any source association must be checked carefully. While the overall 10-sigma detection and 8.6-sigma variability are statistically strong, the orbital modulation remains tentative.

The result also remains a collaboration preprint on the arXiv server, labeled “Submitted to NSR” and not yet marked as peer-reviewed. In modern astrophysics, large teams often share findings first on arXiv for rapid access by the community, but those studies typically undergo additional checks and journal review before the results are treated as established.

Other very-high-energy observatories, including the High-Altitude Water Cherenkov Observatory in Mexico and imaging atmospheric Cherenkov telescopes such as MAGIC and VERITAS, along with the upcoming Cherenkov Telescope Array, will be in a position to test LHAASO’s claim in the coming years.

Whether Cygnus X-3 ultimately secures its place as a confirmed Galactic PeVatron, the LHAASO study underscores how compact, extreme systems in the Milky Way can rival or exceed the energies reached in human laboratories, and points toward a new generation of multi-messenger observations combining gamma rays and neutrinos to trace the origins of cosmic rays.