On November 16, 2025, researchers from the Large High Altitude Air Shower Observatory (LHAASO) announced significant findings that address a long-standing enigma in astrophysics: the peculiar “knee” in the cosmic ray energy spectrum. This knee, identified nearly 70 years ago, marks a sudden decrease in cosmic ray intensity above 3 PeV. The new studies link this phenomenon to micro-quasars, which are believed to be driven by black hole systems, thus providing fresh insights into cosmic ray origins.
For decades, scientists have speculated that the knee represents an acceleration limit of cosmic ray sources, indicating a shift from one power-law distribution of cosmic rays to another. Recent publications in the National Science Review and Science Bulletin detail how micro-quasars act as potent particle accelerators within our galaxy, contributing to the mystery of the knee’s formation.
Micro-Quasars Unveiled as Cosmic Accelerators
Micro-quasars, which occur when black holes pull in material from companion stars, have now been identified as significant contributors to cosmic ray production. LHAASO’s systematic detection of ultra-high-energy gamma rays from five micro-quasars, including SS 433 and Cygnus X-1, marks a pivotal advancement. Notably, the gamma rays from SS 433 were found to overlap with a large atomic cloud, suggesting that high-energy protons are being accelerated by the black hole and colliding with surrounding matter.
The energy produced in these systems is staggering, with proton energies exceeding 1 PeV and total power outputs comparable to that of four trillion hydrogen bombs. Another micro-quasar, V4641 Sgr, reached gamma-ray energies of 0.8 PeV, reinforcing the idea that micro-quasars are indeed capable of accelerating cosmic rays to extreme energies.
Challenges and Innovations in Cosmic Ray Measurement
Understanding the cosmic ray spectrum requires precise measurements, particularly of the lightest nuclei, such as protons. The knee region is sparsely populated with cosmic rays, complicating detection efforts. Traditional satellite detectors face limitations, making the task akin to “finding a needle in a haystack.”
Recently, LHAASO has developed advanced measurement techniques to overcome these challenges. By utilizing a large statistical sample of high-purity protons, researchers achieved precise energy spectrum measurements, rivaling those obtained from satellite experiments. This innovative approach has revealed an unexpected energy spectrum structure that displays a new high-energy component, rather than a straightforward transition between power-law distributions.
The findings suggest that multiple cosmic ray accelerators exist within the Milky Way, each with unique capabilities. The knee reflects the acceleration limit of these sources, primarily indicating that cosmic ray protons in the PeV energy range likely originate from micro-quasars rather than traditional supernova remnants.
This research not only clarifies the origin of the knee but also provides essential observational evidence regarding the role of black holes in cosmic ray production. By connecting the knee structure to micro-quasar systems for the first time, LHAASO has made a significant contribution to our understanding of cosmic ray physics.
The ongoing work at LHAASO, driven by a team from the Chinese Academy of Sciences, Nanjing University, and several international institutions, continues to impact high-energy cosmic-ray research. This research enhances our understanding of extreme physical processes in the universe, shedding light on the intricate mechanisms behind cosmic ray acceleration.
In summary, the discoveries made by LHAASO represent a major milestone in resolving the cosmic ray knee puzzle. With ongoing advancements in detection and measurement, the scientific community is poised to explore further the mysteries of the universe, including the complex interactions between black holes and cosmic rays.
