Research from the Large High Altitude Air Shower Observatory (LHAASO) has resolved a long-standing enigma regarding the cosmic ray energy spectrum, particularly the phenomenon known as the “knee.” This discovery, reported on November 16, 2025, reveals that the knee formation, which shows a pronounced decrease in cosmic rays above 3 Peta-electronvolts (PeV), is linked to micro-quasars powered by black holes.
The knee has perplexed scientists for nearly 70 years, posing questions about the acceleration limits of cosmic ray sources. Historically, it was believed that this transition in the cosmic ray energy spectrum indicated a shift from one power-law distribution to another. Recent studies published in the National Science Review and the Science Bulletin now offer compelling evidence that micro-quasars serve as powerful particle accelerators within our galaxy, contributing to this phenomenon.
Micro-Quasars as Cosmic Ray Accelerators
Micro-quasars are fascinating astrophysical objects formed when black holes draw in material from companion stars in binary systems. The research team, including scientists from the Institute of High Energy Physics at the Chinese Academy of Sciences (CAS), Nanjing University, and La Sapienza University of Rome, 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.
In particular, the findings from SS 433 revealed that ultra-high-energy radiation overlaps with a massive atomic cloud, suggesting that protons are accelerated by the black hole and collide with surrounding matter. The proton energy in this instance exceeded 1 PeV, with a total power output comparable to the energy released by four trillion hydrogen bombs every second. Meanwhile, V4641 Sgr demonstrated gamma-ray energy levels reaching 0.8 PeV, confirming its status as a significant particle accelerator.
Challenges and Innovations in Cosmic Ray Measurement
Understanding the knee’s origins requires precise measurements of cosmic ray energy spectra, particularly for protons, the lightest nuclei. Until recently, the sparse distribution of cosmic rays in the knee region and limitations of satellite detectors made this task seem near impossible. Ground-based measurements often suffer from atmospheric interference, complicating the distinction between protons and other nuclei.
Through innovative techniques, LHAASO has developed advanced measurement capabilities that allow for the selection of a large statistical sample of high-purity protons. This breakthrough has enabled researchers to achieve precise measurements of their energy spectrum, rivaling the accuracy of satellite experiments. The results unveiled a previously unexpected structure in the energy spectrum, characterized by a new high-energy component rather than a straightforward transition between power-law distributions.
The combined findings from LHAASO and data from the space-borne AMS-02 and DArk Matter Particle Explorer (DAMPE) experiments illustrate the existence of multiple cosmic ray accelerators in the Milky Way. Each of these accelerators showcases unique capabilities and energy ranges, with the knee indicating the acceleration limits of sources responsible for generating high-energy cosmic rays.
Ultimately, this research paves the way for a better understanding of cosmic rays and their origins. The observational connection between black holes and cosmic ray phenomena marks a significant advancement in astrophysics, providing crucial insights into the extreme processes at play in the universe. LHAASO’s hybrid detector array, designed and operated by Chinese scientists, continues to lead the way in high-energy cosmic-ray research, contributing to global knowledge of astrophysical processes.
The discoveries not only clarify the mystery surrounding the knee but also enhance our understanding of how black holes influence cosmic ray production, reshaping our perspective of these enigmatic celestial objects.
