LHAASO Discovers the True Origin of the Cosmic Ray "Knee" in the Milky Way
Introduction — A 70-Year Astrophysics Mystery Solved
On 16 November, the Large High Altitude Air Shower Observatory (LHAASO) announced landmark results that resolve a long-standing enigma in astrophysics: the mysterious knee-like dip in the cosmic ray energy spectrum occurring beyond 3 PeV.
The origin of the "knee" has puzzled scientists for almost 70 years. It was long thought to mark the acceleration limit of cosmic-ray sources and signal a shift between two power-law energy distributions.
Now, two new papers—appearing in National Science Review and Science Bulletin—show that micro-quasars powered by black hole accretion are potent particle accelerators within the Milky Way and likely creators of the "knee."
The work was carried out by teams from the Institute of High Energy Physics, Chinese Academy of Sciences (CAS), Nanjing University, the University of Science and Technology of China, La Sapienza University of Rome and several other partners.
Micro-Quasars Identified as Cosmic Accelerators
First Systematic Detection of Ultra-High-Energy Gamma Rays
Black holes—among the universe's most mysterious phenomena—produce relativistic jets when drawing in matter from companion stars in binary systems, creating what are known as "micro-quasars." In this work, LHAASO has, for the first time, systematically detected ultra-high-energy gamma rays from five such system: SS 433, V4641 Sgr, GRS 1915+105, MAXI J1820+070 and Cygnus X-1.
Notably, the ultra-energetic emission from SS 433 aligns with a vast atomic cloud, strongly indicating that protons are accelerated by the black hole and subsequently collide with nearby material.
PeV and Super-PeV Accelerators in the Galaxy
In this system, proton energies surpassed 1 PeV, with a power output of roughly 10³² joules per second—comparable to the energy released each second by four trillion of the world's most powerful hydrogen bombs. Gamma-ray energies from V4641 Sgr reached 0.8 PeV, classing it as another "super-PeV particle accelerator," with parent particle energies exceeding 10 PeV.
These findings confirm that micro-quasars serve as major PeV particle accelerators within the Milky Way, resolving a long-standing scientific puzzle. Although supernova remnants were once regarded as the primary sources of cosmic rays, evidence now shows they cannot propel particles to "knee-level" energies or higher.
Challenges in Measuring Cosmic Ray Spectra
To grasp this phenomenon in full, scientists need highly accurate measurements of the energy spectra of different cosmic-ray species, including the positions of their respective "knees." The starting point is the spectrum of the lightest nuclei—protons. Yet cosmic rays in the knee region are exceedingly rare, and satellite detectors have limited coverage, making detection a near-impossible task.
Ground-based observations face atmospheric interference, which obscures the distinction between protons and heavier nuclei—long thought to make such measurements unattainable.
In this study, LHAASO used its world-leading observational facilities and developed multi-parameter analysis methods, enabling the selection of a large, high-purity sample of protons. This allowed for proton energy spectrum measurements with satellite-level precision, uncovering an entirely unexpected feature: a new "high-energy component" rather than a simple power-law transition.
Multiple Accelerators Shape Cosmic Ray Origins
LHAASO's latest observations, combined with the low-energy readings from the space-borne AMS-02 mission and the mid-energy data gathered by DAMPE, point to the presence of multiple particle accelerators within the Milky Way, each with its own characteristic energy range and acceleration strength. The "knee" appears to mark the upper energy boundary of the sources producing the high-energy component.
The intricate pattern revealed in the proton energy spectrum suggests that PeV-level cosmic ray protons largely stem from "new sources", particularly micro-quasars, whose acceleration power far surpasses that of traditional supernova remnants. As a result, they are capable of producing cosmic rays that extend beyond the "knee."
Breakthrough Connect Black Holes to Cosmic Rays
These two discoveries reinforce one another, creating a cohesive scientific narrative. They not only represent a major step towards resolving the decades-old mystery surrounding the origin of the "knee", but also provide vital observational evidence clarifying the role black holes play in generating cosmic rays.
LHASSO's hybrid detector array, capable of tracing cosmic ray sources via ultra-high-energy gamma rays while simultaneously measuring particles near the solar system, offers a fresh window into PeV-level acceleration and the spectral signatures of these sources. For the first time, the "knee" has been conclusively linked to a defined astrophysical origin—a black hole jet system.
Designed, built and operated entirely by Chinese scientists, LHAASO has become a global leader in high-energy cosmic-ray research thanks to its remarkable sensitivity in gamma-ray astronomy and precision particle detection. Its groundbreaking discoveries continue to expand our understanding of extreme cosmic phenomena.
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