An aerial drone photo taken on October 28, 2025 shows a view of the Large High Altitude Air Shower Observatory (LHAASO) in Daocheng county, Southwest China’s Sichuan Province. Photo: Xinhua
A joint team of Chinese and international scientists released the latest scientific results from China’s Large High Altitude Air Shower Observatory (LHAASO) on Sunday. The data solved a decades-old mystery about the cosmic ray energy spectrum, which shows a sharp decrease—or a “knee”—above 3 peta-electronvolts (PeV).
The cause of the “knee” has remained unclear since its discovery nearly 70 years ago. Scientists have speculated that it is linked to the acceleration limit of the astrophysical sources of cosmic rays and reflects the transition of the cosmic ray energy spectrum from one power-law distribution to another.
Two studies – published in National Science Review and Science Bulletin, respectively – demonstrate that micro-quasars driven by black hole system accretion are powerful particle accelerators in the Milky Way and are the likely source of the “knee.” The studies also advance the understanding of the extreme physical processes of black hole systems, the Global Times learned on Sunday from the Institute of High Energy Physics, Chinese Academy of Sciences (CAS).
Black holes, among the most enigmatic objects in the universe, generate near light-speed jets when pulling in material from companion stars in binary systems, forming micro-quasars. In this study, LHAASO systematically detected ultra-high-energy gamma rays from five micro-quasars for the first time.
Results suggest that the primary cosmic rays generating these gamma rays may have energies exceeding 1 PeV or even 10 PeV— surpassing the energy threshold of the “knee” region. This finding addressed the long-standing issue that the previously recognized cosmic ray sources, such as supernova remnants, couldn’t surpass this threshold either observationally or theoretically.
To further understand the phenomenon, precise measurements of the energy spectra of various cosmic ray species and their respective “knee” regions are essential. However, due to the rarity of cosmic rays in the “knee” region and atmospheric interference, distinguishing protons from other nuclei was very challenging and once considered impossible.
Based on LHAASO’s multiparameter measurement techniques, the research team obtained a large statistical sample of high-purity protons with precision comparable to that of satellite experiments. This measurement revealed a new structure in the energy spectrum that was entirely unexpected, clearly displaying a new “high-energy component.”
LHAASO’s new findings, together with the low-energy component and the intermediate-energy component measured by the space-borne experiments, revealed the existence of multiple accelerator populations within the Milky Way, with each possessing its own unique acceleration capability and energy range. The “knee” represents the acceleration limit of the sources responsible for generating the high-energy component.
These two discoveries support each other, presenting a comprehensive scientific picture. This not only marks a significant advancement in resolving the long-standing mystery of the “knee” origin, but also offers crucial observational evidence for understanding the role of black holes in the origin of cosmic rays.
According to Cao Zhen, an academician of the CAS and chief scientist of LHAASO, the facility’s hybrid detector array design allows for the detection of cosmic ray sources through ultra-high-energy gamma rays, while enabling precise measurement of cosmic ray particles in the vicinity of the solar system.
For the first time, the “knee” structure has been observationally connected to a specific type of astrophysical source – a black hole jet system. It is estimated there are a dozen or so such sources in our Galaxy, Cao told the Global Times on Sunday.
Cao added that the working principles of some artificial accelerators are similar to those of cosmic ray accelerations, and understanding these mechanisms has the potential to be applied in the next generation particle accelerators.
LHAASO, a ground-based cosmic ray observatory designed, constructed and operated by Chinese scientists, is located at a high altitude of 4,410 meters on Mount Haizi in Daocheng county, Southwest China’s Sichuan Province. It has taken the lead in high-energy cosmic ray research due to its sensitivity in both gamma-ray astronomical exploration and cosmic ray precision measurement, and has achieved a series of discoveries with global impact, contributing to human’s understanding of the extreme physical processes in the universe.
(Web editor: Zhang Kaiwei, Liang Jun)