Scientists Uncover Origin of Cosmic Ray ‘Knee’ Linked to Black Holes

Milestones achieved by the Large High Altitude Air Shower Observatory (LHAASO) have shed light on a long-standing mystery regarding the cosmic ray energy spectrum. On November 16, 2025, researchers published findings that link the formation known as the “knee”—a sharp decrease in cosmic ray intensity above 3 PeV—to black hole systems acting as powerful particle accelerators. This connection offers significant insight into the complex nature of cosmic rays, which have puzzled scientists since the knee’s discovery nearly 70 years ago.

The “knee” phenomenon has been attributed to a transition in the cosmic ray energy spectrum from one power-law distribution to another, suggesting an acceleration limit imposed by astrophysical sources. Recent studies published in National Science Review and Science Bulletin reveal that micro-quasars, which are formed when black holes accrete material from companion stars, are likely responsible for this knee structure.

New Research Identifies Black Holes as Cosmic Ray Sources

The research team comprised scientists from the Institute of High Energy Physics of the Chinese Academy of Sciences (CAS), Nanjing University, the University of Science and Technology of China, and La Sapienza University of Rome. They systematically detected ultra-high-energy gamma rays from five identified micro-quasars: SS 433, V4641 Sgr, GRS 1915+105, MAXI J1820+070, and Cygnus X-1. Notably, the emissions from SS 433 were found to coincide with a giant atomic cloud, indicating that high-energy protons are likely accelerated by the black hole and collide with surrounding matter.

The energy levels detected were remarkable, with the protons in SS 433 exceeding 1 PeV. The total power output recorded was approximately 10^32 joules per second, equivalent to the energy released by four trillion of the most powerful hydrogen bombs. V4641 Sgr was also identified as a significant source, producing gamma rays that reached 0.8 PeV.

These findings challenge the traditional view that supernova remnants are the primary sources of cosmic rays. While historically recognized as such, both observational and theoretical evidence suggests that they cannot accelerate cosmic rays to the energies associated with the knee.

Challenges in Cosmic Ray Measurement and New Techniques

Understanding the knee requires precise measurements of the energy spectra of various cosmic ray species, particularly protons. However, cosmic rays in this knee region are sparse, and the limitations of satellite detectors complicate detection efforts. Current methods have made it challenging to distinguish protons from other nuclei, often likening the task to searching for a needle in a haystack.

LHAASO’s advanced observational equipment allowed researchers to develop multi-parameter measurement techniques, resulting in a large statistical sample of high-purity protons. This innovative approach enabled precise measurements of their energy spectrum, achieving a level of accuracy comparable to satellite observations. The results revealed a previously unexpected energy spectrum structure, showcasing a new “high-energy component” rather than a simple transition between power-law distributions.

The findings from LHAASO, combined with data from the AMS-02 experiment and the DArk Matter Particle Explorer (DAMPE), indicate the presence of multiple cosmic ray accelerators within the Milky Way. Each source exhibits unique acceleration capabilities and energy ranges, suggesting that the knee reflects the acceleration limit of the sources generating the high-energy cosmic ray component.

These revelations indicate that cosmic ray protons in the PeV energy range primarily originate from micro-quasars, which possess acceleration limits that exceed those of supernova remnants. This allows them to generate cosmic rays that surpass the knee threshold.

The two studies present complementary findings, collectively advancing the understanding of cosmic rays and their origins. For the first time, the knee structure has been observationally linked to specific astrophysical sources, namely black hole jet systems.

LHAASO’s hybrid detector array is at the forefront of cosmic ray research, successfully detecting sources through ultra-high-energy gamma rays while enabling precise measurements of cosmic ray particles in the vicinity of the solar system. This research not only resolves a significant scientific mystery but also enhances our understanding of the extreme physical processes associated with black holes.

As LHAASO continues to lead in high-energy cosmic-ray research, its findings contribute substantially to the global scientific community’s comprehension of the universe’s most enigmatic phenomena. The implications of these discoveries extend beyond theoretical knowledge, providing crucial observational evidence for the role of black holes in the genesis of cosmic rays.