The Large High Altitude Air Shower Observatory (LHAASO) has made a breakthrough in exploring the extreme universe. For the first time, the LHAASO collaboration has detected ultra-high-energy (UHE) gamma rays—with energies exceeding 100 trillion electron-volts (TeV)—from a gamma-ray binary system, LS I +61° 303. The discovery challenges existing theories of particle acceleration in extreme astrophysical environments.
The study was published online in Physical Review Letters on April 30. It was selected as an Editor's Suggestion and was also featured as a Synopsis by Physics Magazine. This study was led by researchers from the Institute of High Energy Physics (IHEP) of the Chinese Academy of Sciences (CAS), along with their collaborators from the Shanghai Astronomical Observatory of CAS and other institutions.
The origin of high-energy cosmic rays—charged particles from outer space—remains a "century-old puzzle" in astrophysics. Searching for extreme particle accelerators capable of accelerating particles to the peta-electron-volt (PeV, 1,000 trillion electron-volts) level is crucial to solving this mystery.
Gamma-ray binaries, composed of a massive star and a compact star (either a neutron star or a stellar-mass black hole), are potential cosmic-ray accelerators and serve as natural laboratories for extreme physics. However, only a handful of binary systems are known to emit very-high-energy (VHE, > 0.1 TeV) gamma rays. LS I +61° 303—a classical gamma-ray binary—had previously been studied with energy up to approximately 10 TeV. Scientists didn't know whether it could accelerate particles to higher energies.
Taking advantage of LHAASO's exceptional sensitivity and broad energy coverage, the LHAASO collaboration measured the energy spectrum of LS I +61° 303 up to 200 TeV—confirming it to be a UHE gamma-ray binary. Furthermore, the collaboration discovered that the flux of the system—the brightness of its gamma rays—varies with its orbital period of about 26.5 days, and this "orbital modulation" exhibited a clear energy dependence, revealing complex internal physical processes.
In binary systems, the strong magnetic field causes high-energy electrons to rapidly lose energy via synchrotron radiation, preventing electrons from being accelerated to the UHE range. Therefore, the detected photons above 100 TeV strongly suggest that, during specific orbital phases of the system, high-energy protons (hadrons) are accelerated and collide with the dense surrounding stellar wind, producing these UHE gamma rays.
This discovery provides critical evidence that gamma-ray binaries like LS I +61° 303 are potential PeVatrons—capable of accelerating cosmic rays to PeV energies. This work imposes new, stringent constraints on theoretical models of particle acceleration and radiation in extreme astronomical environments, paving the way for future "multi-messenger astronomy" that relies on both electromagnetic and non-electromagnetic signals.
