A significant breakthrough in astrophysics was announced on November 16, 2025, as researchers from the Large High Altitude Air Shower Observatory (LHAASO) revealed the origins of a long-standing mystery regarding cosmic rays. The studies published in the National Science Review and Science Bulletin suggest that the “knee” formation in the cosmic ray energy spectrum, characterized by a sharp decline in cosmic rays above 3 PeV, is likely linked to black holes acting as powerful particle accelerators within the Milky Way.
The “knee” phenomenon, first identified nearly 70 years ago, had puzzled scientists who speculated that it indicated the limits of acceleration from cosmic ray sources. Now, the revelations indicate that micro-quasars—binary systems where a black hole accretes material from a companion star—are central to understanding this energy spectrum transition. The research involved collaboration among scientists from the Institute of High Energy Physics of the Chinese Academy of Sciences, Nanjing University, and La Sapienza University of Rome, among others.
Unveiling Micro-Quasars as Cosmic Accelerators
Micro-quasars, which produce relativistic jets while pulling material from companion stars, have been identified as crucial cosmic accelerators. The LHAASO study reported the detection of 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 radiation from SS 433 coincided with a massive atomic cloud, suggesting that protons are accelerated by the black hole and collide with surrounding matter.
The energy levels observed were remarkable, with protons in the SS 433 system exceeding 1 PeV and an output power of about 10^32 joules per second—equivalent to the energy released by four trillion hydrogen bombs. V4641 Sgr was also noted for producing gamma rays reaching 0.8 PeV, further highlighting the capability of these micro-quasars to accelerate particles to such extreme energies.
Challenges and Innovations in Cosmic Ray Measurement
Understanding the cosmic ray spectrum requires precise measurements of various cosmic ray species, particularly protons. Traditionally, identifying cosmic rays in the “knee” region has been challenging due to the sparse nature of these particles and the limitations of satellite detectors. Ground-based measurements face atmospheric interference, complicating the distinction between protons and other nuclei.
Utilizing its advanced observational equipment, LHAASO developed multi-parameter measurement techniques that enabled the collection of a large statistical sample of high-purity protons. This innovation allowed researchers to achieve energy spectrum measurements with precision comparable to satellite experiments, revealing an unexpected energy spectrum structure. Instead of a simple transition between power-law spectra, a new “high-energy component” emerged.
The findings from LHAASO, combined with low-energy measurements from the AMS-02 experiment and intermediate-energy data from the DArk Matter Particle Explorer (DAMPE), indicate that multiple accelerators exist within the Milky Way. Each source demonstrates unique capabilities in cosmic ray acceleration, suggesting that the “knee” marks the limits of acceleration for these high-energy components.
The discoveries not only enhance our understanding of the origins of cosmic rays but also strengthen the connection between black holes and cosmic ray acceleration. LHAASO’s hybrid detector array has played a pivotal role in this research by enabling precise cosmic ray measurements and detection of ultra-high-energy gamma rays.
With these advancements, researchers have taken a significant step towards resolving the mystery of the “knee” and elucidating the role of black holes in cosmic ray dynamics. The implications of these findings may reshape our understanding of the universe’s most extreme physical processes, showcasing the potential of black holes as key cosmic players.