Milestones achieved by the Large High Altitude Air Shower Observatory (LHAASO) on November 16, 2025, have clarified a longstanding enigma concerning cosmic rays. For nearly 70 years, scientists have puzzled over the “knee” in the cosmic ray energy spectrum—an abrupt drop in cosmic ray intensity above 3 PeV that suggests a transformative shift in energy distribution. Recent studies published in the National Science Review and Science Bulletin identify micro-quasars associated with black holes as critical accelerators responsible for this phenomenon.
Researchers from the Institute of High Energy Physics at the Chinese Academy of Sciences (CAS), alongside teams from Nanjing University, the University of Science and Technology of China, and La Sapienza University of Rome, have made significant advances in understanding the role of these micro-quasars in the Milky Way.
Micro-Quasars as Cosmic Accelerators
Micro-quasars, which form when black holes accrete material from companion stars, produce relativistic jets that can accelerate particles to extreme energies. LHAASO’s recent observations have detected ultra-high-energy gamma rays from five notable micro-quasars: SS 433, V4641 Sgr, GRS 1915+105, MAXI J1820+070, and Cygnus X-1. Specifically, the findings from SS 433 revealed an impressive proton energy exceeding 1 PeV, with an energy output equivalent to that of four trillion hydrogen bombs.
The study highlighted that gamma-ray emissions from V4641 Sgr reached up to 0.8 PeV, marking it as another significant particle accelerator. These findings challenge the previous understanding that supernova remnants were the primary sources of cosmic rays, as they cannot produce particles with energies high enough to account for the knee.
Challenges in Cosmic Ray Measurement
Understanding the cosmic ray spectrum requires precise measurements of various cosmic ray species, particularly protons. However, cosmic rays in the knee region are relatively rare, and satellite detectors struggle to capture them effectively. Ground-based measurements face atmospheric interference, complicating the differentiation between protons and other nuclei.
Leveraging its advanced observational capabilities, LHAASO developed innovative multi-parameter measurement techniques. This allowed researchers to compile a large, high-purity sample of protons, enabling them to measure the energy spectrum with precision comparable to satellite experiments. The results revealed an unexpected energy spectrum structure, showcasing a new “high-energy component” instead of a straightforward 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 multiple accelerators within the Milky Way, each with unique capabilities. The knee in the energy spectrum indicates the acceleration limit of the sources contributing to the high-energy component.
The research underscores that cosmic ray protons in the PeV range largely originate from micro-quasars, which have a significantly higher acceleration limit than supernova remnants. This capability enables them to produce cosmic rays that surpass the knee.
The comprehensive insights provided by these studies not only enhance our understanding of cosmic ray origins but also solidify the connection between black holes and cosmic rays. For the first time, the knee structure has been directly linked to black hole jet systems, offering a clearer picture of how these enigmatic objects influence the cosmos.
As LHAASO continues to lead in high-energy cosmic-ray research, its hybrid detector array remains pivotal in uncovering the complexities of cosmic ray origins and the extreme physical processes at play in the universe. The ongoing discoveries promise to reshape our understanding of astrophysics and the role of black holes in the broader cosmic landscape.
