Identifying a New Cosmic-Ray Accelerator
Cosmic rays with energies up to 3–4 peta-electron-volts (1015 eV) were long thought to originate largely from supernovae within our Galaxy. Recent measurements by the Large High Altitude Air Shower Observatory (LHAASO) in China revealed a surprisingly complex spectrum around that energy, hinting at additional types of cosmic-ray accelerators (see Synopsis: Elucidating the Cosmic-Ray Knee). Now the LHAASO team has found that some of these accelerators could be so-called gamma-ray binaries—systems comprising one massive star and one neutron star or black hole [1].
LHAASO’s detectors are sensitive to particle cascades created when gamma rays and cosmic rays strike the atmosphere. Gamma rays are distinguished from the much more numerous cosmic rays in that they are far less likely to generate muons. Over a three-year period, LHAASO detected 16 gamma-ray photons whose energies exceeded 100 tera-electron-volts (TeV) from the gamma-ray binary LS I +61° 303. The system’s previously measured emission topped out at around 20 TeV and was seen to peak when the binary’s neutron star was farthest from its companion star, whereas the higher-energy emission recorded by LHAASO rose when the two stars were closest.
The LHAASO researchers speculate that this energy-dependent modulation indicates two distinct mechanisms. The lower-energy gamma rays are likely produced when relativistic electrons—accelerated by shockwaves where the stars’ winds collide—energize photons from the companion star. Thanks to a trade-off between electron-cooling rates and photon density, this process is more efficient at larger orbital separations. The higher-energy gamma rays are likely produced when relativistic protons—also accelerated by the shockwaves—interact with the companion star’s dense wind. The greater-than-100-TeV energy of these gamma rays implies that the originating protons reach peta-electron-volt energies and suggests that LS I +61° 303 and systems like it could be accelerators of ultrahigh-energy cosmic rays.
–Marric Stephens
Marric Stephens is a Corresponding Editor for Physics Magazine based in Bristol, UK.
References
- Z. Cao et al. (LHAASO Collaboration), “First detection of ultrahigh energy emission from gamma-ray binary LS I +61° 303,” Phys. Rev. Lett. 136, 181001 (2026).