Milestone findings released by the Large High Altitude Air Shower Observatory (LHAASO) on November 16, 2025, have provided answers to a decades-old puzzle regarding the cosmic ray energy spectrum. Scientists have long observed a sharp decrease in cosmic rays above 3 PeV, creating a distinctive knee-like shape in the spectrum. The cause of this phenomenon, known as the “knee,” has eluded researchers since its identification nearly 70 years ago.
Recent studies published in National Science Review and Science Bulletin reveal that micro-quasars, which are black hole systems that accrete material from companion stars, serve as powerful particle accelerators in the Milky Way and are likely responsible for the formation of this knee.
Micro-Quasars as Cosmic Accelerators
Black holes, among the most mysterious objects in the universe, generate relativistic jets when they draw in material from nearby stars, forming what are known as micro-quasars. The LHAASO team systematically detected ultra-high-energy gamma rays from five notable micro-quasars: SS 433, V4641 Sgr, GRS 1915+105, MAXI J1820+070, and Cygnus X-1. Notably, ultra-high-energy emissions from SS 433 were found to coincide with a large atomic cloud, indicating that high-energy protons are accelerated by the black hole and collide with surrounding matter.
In this system, proton energies exceeded 1 PeV, with a total power output of approximately 10^32 joules per second. This energy output is equivalent to the power released every second by four trillion of the most powerful hydrogen bombs. In another significant finding, the gamma-ray energy from V4641 Sgr reached 0.8 PeV, establishing it as another “super PeV particle accelerator.” The parent particles generating these gamma rays had energies that exceeded 10 PeV.
These results confirm that micro-quasars are significant sources of PeV particle acceleration within our galaxy, addressing a critical question in astrophysics. Historically, supernova remnants were credited as cosmic ray sources, but both observational and theoretical studies indicate that they cannot accelerate cosmic rays to the energies associated with the knee.
Challenges and Advances in Cosmic Ray Measurement
To fully understand cosmic rays, precise measurements of energy spectra for various cosmic ray species, including their respective knees, are essential. The initial step involves measuring the energy spectrum of the lightest nuclei—protons. However, cosmic rays within the knee region are sparse, complicating detection efforts.
Satellite detectors have limited acceptance, making it challenging to isolate protons from other nuclei. For some time, the measurement of these energies was deemed nearly impossible. Leveraging its advanced ground-based cosmic ray observational equipment, LHAASO developed multi-parameter measurement techniques and compiled a large statistical sample of high-purity protons. This allowed for a precise measurement of their energy spectrum, achieving accuracy comparable to satellite experiments.
The LHAASO measurements revealed a previously unanticipated energy spectrum structure, showcasing a new high-energy component rather than a simple transition between power-law spectra. This indicates that cosmic ray protons in the PeV energy range are primarily sourced from “new sources,” such as micro-quasars, which have an acceleration limit significantly higher than that of supernova remnants.
The findings from LHAASO, combined with low-energy data from the space-based AMS-02 experiment and intermediate-energy data from the DArk Matter Particle Explorer (DAMPE) experiment, illustrate the existence of multiple accelerators within the Milky Way. Each source possesses distinct acceleration capabilities and energy ranges, with the knee marking the acceleration limit of those sources generating the high-energy component.
These discoveries offer a comprehensive understanding of the origins of cosmic rays, linking them to black hole jet systems for the first time. LHAASO’s innovative hybrid detector array design enables the detection of cosmic ray sources through ultra-high-energy gamma rays while facilitating precise measurements of cosmic ray particles near the solar system.
The significant advancements made by LHAASO not only resolve the longstanding mystery of the knee’s origin but also provide valuable observational evidence for the role of black holes in cosmic ray generation. As the observatory continues to lead high-energy cosmic-ray research, its contributions enhance our understanding of the extreme physical processes occurring throughout the universe.
