A significant breakthrough in astrophysics has emerged as scientists have linked the formation of the cosmic ray “knee” to black hole systems. This revelation came from recent findings announced by the Large High Altitude Air Shower Observatory (LHAASO) on November 16, 2025, which address a mystery that has puzzled researchers for nearly seven decades.
The “knee” refers to a notable drop in the energy spectrum of cosmic rays beyond 3 PeV, a phenomenon first identified in the 1950s. For years, scientists have speculated that this knee shape indicates an acceleration limit of cosmic ray sources, suggesting a transition between different energy distributions. New research published in the National Science Review and Science Bulletin indicates that micro-quasars—systems where black holes accrete material from companion stars—are likely responsible for producing this knee effect.
This monumental research effort involved collaboration among institutions including the Institute of High Energy Physics of the Chinese Academy of Sciences (CAS), Nanjing University, and La Sapienza University of Rome.
Micro-Quasars as Cosmic Accelerators
Micro-quasars, which generate relativistic jets while accreting material, have been identified as powerful particle accelerators. In a groundbreaking study, 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.
The ultra-high-energy radiation emitted from SS 433 was particularly noteworthy as it coincided with a giant atomic cloud, suggesting that protons are being accelerated by the black hole before colliding with surrounding matter. Measurements indicated that the energy of protons in this system exceeded 1 PeV, with a power output comparable to that of four trillion hydrogen bombs every second. V4641 Sgr also exhibited gamma-ray energies reaching 0.8 PeV, categorizing it as another significant particle accelerator.
These findings confirm that micro-quasars play a critical role in producing high-energy cosmic rays, challenging the long-held belief that supernova remnants were the primary sources of such phenomena.
Challenges in Cosmic Ray Measurement
Understanding the cosmic ray spectrum requires precise measurements, particularly of the lightest nuclei, such as protons. The knee region of cosmic rays is notably sparse, complicating detection due to the limitations of satellite detectors. Ground-based measurements are hindered by atmospheric interference, further complicating efforts to differentiate protons from other particles.
In their research, LHAASO utilized advanced ground-based observational technology, employing multi-parameter measurement techniques to compile a large dataset of high-purity protons. This approach yielded precise measurements of the energy spectrum, matching the accuracy of satellite experiments. Notably, the results revealed an unexpected structure in the energy spectrum, highlighting a new “high-energy component” rather than a simple transition between power-law spectra.
The collaboration among LHAASO and other observational experiments, such as the AMS-02 and DArk Matter Particle Explorer (DAMPE), has unveiled a complex picture of cosmic ray origins. These studies point to multiple accelerators within the Milky Way, each with distinct acceleration capabilities. The knee phenomenon reflects the limits of the sources that contribute to this high-energy component, suggesting that micro-quasars have a significantly higher acceleration capacity than supernova remnants.
The findings not only clarify the origin of the knee structure but also enhance our understanding of black holes’ roles in cosmic ray production. The connection between the knee and specific astrophysical sources offers new insights into the extreme processes occurring in the universe.
LHAASO’s innovative design and operational capabilities have positioned it at the forefront of high-energy cosmic-ray research, enabling researchers to make substantial contributions to our understanding of the universe’s most enigmatic phenomena. As this field of study continues to evolve, the implications of these findings will undoubtedly resonate across various scientific disciplines.
