A significant breakthrough in astrophysics has occurred as researchers have successfully linked the formation of the “knee” in the cosmic ray energy spectrum to black holes. On November 16, 2025, the Large High Altitude Air Shower Observatory (LHAASO) unveiled findings that clarify a decades-old enigma regarding cosmic rays, highlighting their distinctive drop in intensity above 3 PeV.
The “knee” phenomenon, first identified nearly 70 years ago, has long puzzled scientists. Historically, it was thought to represent a transition in cosmic ray energy sources, but recent research indicates that black holes may play a pivotal role in this process. The studies published in the National Science Review and Science Bulletin suggest that micro-quasars—systems where black holes pull in material from companion stars—act as powerful particle accelerators in the Milky Way.
Micro-Quasars Identified as Key Cosmic Accelerators
Micro-quasars generate relativistic jets during their accretion process, and LHAASO has made groundbreaking observations by detecting 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 findings from SS 433 revealed a substantial overlap with a massive atomic cloud, suggesting that high-energy protons are accelerated by the black hole before colliding with surrounding matter.
In terms of energy, the protons in this system exceeded 1 PeV, generating an astonishing power output of approximately 10^32 joules per second. This output is equivalent to the energy released by four trillion of the most powerful hydrogen bombs. Similarly, V4641 Sgr reached gamma-ray energies of 0.8 PeV, reinforcing its status as a “super PeV particle accelerator.”
These discoveries challenge previous assumptions about cosmic ray sources. While supernova remnants were once considered the primary accelerators, both observational and theoretical studies suggest they cannot account for cosmic rays with energies high enough to create the “knee” in the spectrum.
New Measurement Techniques Overcome Historical Challenges
Understanding the cosmic ray phenomenon requires accurate measurements of energy spectra, particularly for lighter nuclei like protons. However, the “knee” region poses significant detection challenges, as cosmic rays are sparse and satellite detectors have limited capacity. Ground-based measurements also face atmospheric interference, complicating the differentiation of protons from other particles.
In this groundbreaking study, LHAASO utilized its advanced observational technology to develop multi-parameter measurement techniques. By selecting a large statistical sample of high-purity protons, researchers achieved precise measurements comparable to those obtained from satellite experiments. This analysis revealed an unexpected energy spectrum structure, highlighting a new “high-energy component” rather than a mere transition between power-law spectra.
LHAASO’s results, when combined with data from the AMS-02 experiment and the DArk Matter Particle Explorer (DAMPE), indicate the presence of multiple cosmic ray accelerators within the Milky Way. Each source appears to have distinct acceleration capacities and energy ranges. The “knee” signifies the acceleration limit of these high-energy cosmic ray sources, with micro-quasars emerging as significant contributors.
The implications of this research extend beyond mere academic interest. The findings provide critical observational evidence for the role of black holes in cosmic ray origins, enhancing our understanding of the extreme processes occurring in the universe. For the first time, the “knee” has been observationally linked to a specific astrophysical source, namely the black hole jet system.
As LHAASO continues to lead in high-energy cosmic ray research, its innovative hybrid detector array facilitates the detection of cosmic ray sources via ultra-high-energy gamma rays. This enables precise measurements of cosmic ray particles near the solar system, ultimately contributing to a deeper comprehension of the universe’s complex dynamics.
These advancements not only resolve a long-standing scientific mystery but also set the stage for further exploration into the cosmic phenomena that shape our understanding of the universe.
