Scientists Unravel Cosmic Ray Mystery Linked to Black Holes
On November 16, 2025, researchers at the Large High Altitude Air Shower Observatory (LHAASO) announced groundbreaking findings that have resolved a decades-old enigma regarding the cosmic ray energy spectrum. The research reveals that the so-called “knee”—a notable decrease in cosmic rays above 3 PeV—is linked to black holes acting as powerful particle accelerators. This discovery marks a significant advancement in our understanding of cosmic rays and their origins.
The “knee” in the cosmic ray spectrum, first identified nearly 70 years ago, has puzzled scientists. It was believed to indicate an acceleration limit in the astrophysical sources of cosmic rays, suggesting a transition in energy distribution. Recent studies published in the National Science Review and Science Bulletin indicate that micro-quasars, which are black hole systems that accrete material from companion stars, are the likely source of this phenomenon.
Micro-Quasars Identified as Cosmic Accelerators
Black holes, among the most mysterious entities in the universe, can generate relativistic jets when they draw material from nearby stars, forming what are known as micro-quasars. In this latest research, LHAASO systematically 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. Notably, the gamma rays from SS 433 were found to coincide with a giant atomic cloud, indicating that high-energy protons are likely being accelerated by the black hole and subsequently colliding with surrounding matter.
The energy observed in this system exceeded 1 PeV, with a total output of approximately 10^32 joules per second—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, further solidifying the idea that micro-quasars are significant particle accelerators within our galaxy.
Challenges and Advances in Cosmic Ray Research
Understanding the cosmic ray spectrum requires precise measurements of various cosmic ray species and their respective “knees.” The first step is to measure the energy spectrum of protons, the lightest nuclei. However, the scarcity of cosmic rays in the knee region and the limited acceptance of satellite detectors have made this task exceptionally challenging. Traditional ground-based measurements often struggle with atmospheric interference, complicating efforts to distinguish protons from other nuclei.
In a notable advancement, LHAASO utilized its state-of-the-art ground-based observational equipment to develop multi-parameter measurement techniques. This approach allowed for the collection of a large statistical sample of high-purity protons, enabling precise energy spectrum measurements with accuracy comparable to satellite experiments. The results revealed an unexpected energy spectrum structure, showcasing a new “high-energy component” rather than a simple transition between power-law spectra.
These findings, along with data from the space-borne AMS-02 and DArk Matter Particle Explorer (DAMPE) experiments, indicate the presence of multiple cosmic ray accelerators within the Milky Way, each exhibiting unique acceleration capabilities and energy ranges. The knee represents the acceleration limit of these sources, highlighting that cosmic ray protons in the PeV energy range largely originate from newly identified sources like micro-quasars rather than traditional supernova remnants.
These groundbreaking discoveries not only clarify the origins of the knee but also enhance our understanding of black holes’ roles in cosmic ray production. The LHAASO’s hybrid detector array effectively detects cosmic ray sources through ultra-high-energy gamma rays while allowing for precise measurements of cosmic ray particles near the solar system. This research provides valuable insights into the acceleration capabilities of cosmic ray sources at PeV energies.
For the first time, the knee structure has been observationally linked to black hole jet systems, marking a pivotal moment in astrophysics. The work conducted by the Institute of High Energy Physics of the Chinese Academy of Sciences and collaborating institutions underscores the profound impact of these findings on our comprehension of cosmic phenomena. With LHAASO’s advancements, researchers are poised to explore the extreme physical processes that govern our universe.
