Milestone research from the Large High Altitude Air Shower Observatory (LHAASO) has provided critical insights into the enigmatic “knee” formation in the cosmic ray energy spectrum. Released on November 16, 2025, the findings explain the long-standing mystery of why cosmic rays exhibit a sharp decline above 3 PeV. This “knee” shape, first observed nearly 70 years ago, is now linked to black holes, specifically through the activity of micro-quasars, which are powerful particle accelerators within our galaxy.
The knee’s origin has puzzled scientists for decades. Prior theories suggested it might be connected to the limits of cosmic ray sources’ acceleration. Recent studies published in the National Science Review and Science Bulletin have shifted this understanding, indicating that the acceleration processes in micro-quasars are responsible for the knee’s formation. This research was spearheaded by a collaborative effort involving 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.
Micro-Quasars as Key Accelerators
Micro-quasars are fascinating astronomical entities formed when black holes draw in matter from companion stars, generating relativistic jets. In their latest research, LHAASO successfully detected ultra-high-energy gamma rays from five specific micro-quasars: SS 433, V4641 Sgr, GRS 1915+105, MAXI J1820+070, and Cygnus X-1. Notably, the high-energy emissions from SS 433 correlated with a massive atomic cloud, indicating that high-energy protons are propelled by the black hole and collide with surrounding matter.
In this complex environment, protons exceeded energies of 1 PeV, with a staggering total power output equivalent to the energy released by four trillion hydrogen bombs every second. V4641 Sgr was identified as another significant accelerator, producing gamma-ray energy reaching 0.8 PeV, indicating the capability of these micro-quasars to generate high-energy cosmic rays that surpass the knee threshold.
Advancements in Cosmic Ray Measurements
Understanding the cosmic ray energy spectrum requires precise measurement techniques, particularly for light nuclei like protons. Historically, cosmic rays in the knee region have been difficult to measure due to their low abundance and the limitations of satellite detectors. Ground-based indirect measurements face atmospheric interference, complicating the differentiation between protons and other nuclei.
Thanks to LHAASO’s advanced observational equipment, researchers developed multi-parameter measurement techniques, allowing them to analyze a large sample of high-purity protons. This enabled the precise measurement of their energy spectrum, achieving accuracy comparable to satellite observations. The results revealed an unexpected structure in the energy spectrum, displaying a new high-energy component rather than a straightforward transition between power-law distributions.
These findings, combined with data from space-borne experiments like AMS-02 and DArk Matter Particle Explorer (DAMPE), indicate multiple cosmic ray sources within the Milky Way, each with unique acceleration capabilities. The knee serves as a marker for the acceleration limits of these sources, confirming that cosmic ray protons in the PeV energy range predominantly originate from new sources such as micro-quasars, which can generate cosmic rays that exceed the knee’s energy threshold.
The results from LHAASO not only clarify the mystery surrounding the knee in the cosmic ray spectrum but also provide significant observational evidence regarding the role of black holes in cosmic ray origins. The observatory’s innovative hybrid detector array allows for effective detection of cosmic ray sources via ultra-high-energy gamma rays while enabling accurate measurements of cosmic ray particles near the solar system.
These advancements mark a substantial leap in our understanding of the extreme physical processes associated with black holes and their contributions to cosmic ray generation. The collaboration of leading institutions and the pioneering research conducted at LHAASO highlight the growing importance of high-energy cosmic-ray research and its implications for astrophysics on a global scale.
