LHAASO uncovered key clue to the origin of cosmic ray “knee”

The Large High Altitude Air Shower Observatory (LHAASO) released its latest research findings on “Unraveling the mystery of cosmic ray ‘Knee’ Formation” on November 16, 2025. Observations demonstrated that microquasars driven by stellar black hole accretion are powerful particle accelerators within the Milky Way, which is capable of accelerating protons to 1 peta-electronvolt (PeV).1 Additionally, the cosmic ray proton spectrum exhibits an unexpectedly high-energy component around the “knee” region, with microquasars being the most likely candidate sources.2 The research was completed by the LHAASO Collaboration led by Prof. Zhen Cao as the spokesperson from the Institute of High Energy Physics, Chinese Academy of Sciences, which not only shed light on the key mechanisms behind the origin of cosmic rays but also contribute to a deeper understanding of the extreme physical processes around black holes.

Cosmic rays are high-energy particles from space, primarily composed of protons and nuclei, carrying crucial information concerning the origin of the universe, the evolution of celestial bodies, solar activities, as well as Earth’s space environment. Although the study of cosmic rays can be dated back to the pioneering discovery by Austrian physicist Victor F. Hess in 1912, the discipline remains relatively “young” and still lacks a “standard model” capable of comprehensively describing the origins, acceleration mechanisms, and propagation of cosmic rays through interstellar space. The origin of cosmic rays is regarded as the main problem in cosmic ray research. The celestial sources of cosmic rays have not been identified and the mechanisms by which the cosmic rays are accelerated to energies millions of times greater than those achieved by artificial accelerators remain unclear. This is primarily due to the pervasive magnetic field in the universe, which causes charged cosmic ray particles to lose information about their origin during their long-distance propagation. Consequently, neutral particles, especially gamma rays and neutrinos, are considered the most promising candidates for probing the origin of cosmic rays.

Chinese scientists have been systematically exploring cosmic rays since the 1950s. In 1954, China’s first high-altitude cosmic ray laboratory was built on Luoxue Mountain in Dongchuan, Yunnan Province, at an altitude of 3,180 m. This laboratory contributed to studying electromagnetic cascades and strange particles. Subsequently, the large cloud chamber at Yunnan Cosmic Ray Station and emulsion chambers installed at Kanbala Mountain (5,500 m above sea level [a.s.l.]) in Tibet were established. Furthermore, the China-Japan cosmic ray experiment ASγ was built at Yangbajing, Tibet, located at an altitude of 4,300 m, in 1989, which took the advantages of high altitude and the combination of experiments of extensive air shower array, emulsion chambers and burst detectors, to study the primary energy spectrum and composition of ultra-high-energy (UHE) cosmic ray. While the China-Italy ARGO-YBJ experiment was built at the same site in 2006, designed to detect small size air showers at an energy threshold of ∼100 GeV using a full coverage resistive plate chamber array. Both laid a solid foundation for China’s further development in the observational research of gamma-ray astronomy.