In a groundbreaking development that promises to transform our understanding of cosmic ray origins, the Large High Altitude Air Shower Observatory (LHAASO) has unveiled pivotal new findings that resolve a decades-old enigma in astrophysics. This mystery centers on the perplexing “knee” feature in the cosmic ray energy spectrum, a sharp decline in flux observed above energies of approximately 3 petaelectronvolts (PeV). Since its initial detection nearly seventy years ago, the origin of this knee structure has baffled scientists, sparking numerous theories about the astrophysical mechanisms capable of accelerating particles to such extraordinary energies.
The knee in the cosmic ray spectrum has long been hypothesized as a signature of the limit to which conventional cosmic accelerators could energize particles, marking a transition from one power-law regime to another. Yet, definitive evidence connecting this feature to specific astrophysical sources remained elusive—until now. Published in leading journals National Science Review and Science Bulletin, two comprehensive studies led by an international consortium of researchers have identified micro-quasars powered by black hole accretion systems as the prime candidates behind this cosmic phenomenon. These compact high-energy sources operate within the Milky Way and produce particle acceleration previously underestimated in galactic models.
Micro-quasars are formed when black holes in binary star systems siphon material from their companions, triggering relativistic jets capable of accelerating particles to ultra-high energies. Utilizing the unparalleled sensitivity and hybrid detection capabilities of LHAASO, scientists achieved the first systematic observation of ultra-high-energy gamma rays emanating from five notable micro-quasars: SS 433, V4641 Sgr, GRS 1915+105, MAXI J1820+070, and Cygnus X-1. These gamma emissions provide a direct window into the presence of PeV-energy protons and other cosmic particles energized by black hole jet systems.
Especially striking were the findings related to SS 433, where gamma radiation spatially coincides with a massive ambient atomic cloud. This overlap signifies that particles accelerated by the black hole are interacting with surrounding matter, thereby producing gamma rays via hadronic collisions. Intriguingly, the proton energies in SS 433 exceed 1 PeV, with an astounding power output on the order of 10^32 joules per second—energy comparable to the detonation of four trillion powerful hydrogen bombs every second. Similarly, V4641 Sgr exhibited gamma rays reaching 0.8 PeV, categorizing it as a super PeV particle accelerator with parent particles surpassing 10 PeV in energy.
These discoveries profoundly challenge the traditional paradigm that supernova remnants are the exclusive sources of galactic cosmic rays. While supernova remnants do contribute to cosmic ray acceleration, both observational data and theoretical models assert their incapacity to reach energies beyond the knee. The LHAASO observations reposition micro-quasars as vital agents capable of not only reaching but exceeding the knee threshold, thereby filling a critical gap in cosmic ray origin theories.
A major technical hurdle overcome by LHAASO was the precise measurement of the proton energy spectrum in the PeV region. Detecting cosmic ray protons at these extreme energies is inherently difficult due to their rarity and the interference introduced by the Earth’s atmosphere in ground-based observatories. Furthermore, satellite-based cosmic ray detectors possess limited collection areas, restricting their effectiveness at ultra-high energies. Through a novel multi-parameter measurement approach and rigorous statistical selection of a high-purity proton sample, LHAASO achieved precision rivaling satellite measurements, revealing a complex spectral structure characterized by an unanticipated high-energy component superimposed on existing power-law distributions.
Integrating LHAASO’s findings with those from space-borne experiments such as AMS-02 and DAMPE, scientists have recognized multiple, discrete acceleration sources within the Milky Way, each imprinting distinct spectral features on cosmic rays. This multiplex acceleration model elucidates the knee as the energy boundary of the most potent accelerators—in this case, black hole-driven micro-quasars—rather than a singular universal cutoff. This nuance significantly redefines the cosmic ray landscape and provides essential context for interpreting high-energy particle flux variations observed on Earth.
The enriched understanding emerging from these studies emphasizes that the cosmic ray proton flux in the PeV range is dominated by contributions from micro-quasars, with acceleration capabilities far surpassing those of supernova remnants. This hierarchical structuring implies that while supernova remnants provide the bulk of lower energy cosmic rays, micro-quasars furnish the extreme high-energy particles responsible for the knee, thereby offering a more comprehensive explanation compatible with observational spectra.
Together, the discoveries of micro-quasar gamma-ray emissions and the refined proton energy spectrum measurement combine into a compelling narrative that not only resolves the knee’s origin but also illuminates the role of black holes as astrophysical particle accelerators. This breakthrough provides a critical observational anchor for theoretical models that have long suggested relativistic jets as engines of extreme particle acceleration, cementing black hole systems as fundamental contributors to the high-energy cosmic environment.
LHAASO’s multidetector, hybrid array methodology, which simultaneously observes cosmic ray sources through ultra-high-energy gamma rays and performs localized cosmic ray particle measurements near Earth, represents a revolutionary approach. This dual capacity allows unprecedented cross-validation of acceleration processes and spectral signatures, linking distant astrophysical phenomena with terrestrial detection data. For the first time, observational evidence directly associates the knee structure with a distinct category of astrophysical accelerator—black hole jet systems—sharpening our ability to map cosmic origins.
Conceived, designed, and operated by Chinese scientists, LHAASO embodies cutting-edge technology in high-energy particle astrophysics. Its sensitivity to both gamma-ray astronomy and cosmic ray measurement has enabled a suite of globally impactful discoveries over recent years, pushing the frontiers of knowledge regarding extreme physical processes in the universe. With these latest contributions, LHAASO cements its position at the forefront of cosmic ray research, enabling deeper exploration into the mechanisms shaping the high-energy cosmos.
Moreover, the implications of this research extend beyond cosmic ray physics, touching fundamental questions about black hole accretion, jet formation, and particle acceleration mechanisms under extreme gravitational and magnetic fields. As micro-quasars now emerge as key astro-particle laboratories, further observations will refine particle acceleration models and potentially unravel connections between cosmic rays and other high-energy astrophysical phenomena such as neutrinos and gravitational waves.
This paradigm-shifting work underscores the power of international collaboration and state-of-the-art observatories in resolving astrophysical puzzles once considered intractable. As cosmic ray research progresses, the integration of multi-messenger astronomy and next-generation instrumentation will undoubtedly build upon the foundation established by LHAASO’s groundbreaking findings, ushering a new era of discovery in high-energy astrophysics.
Subject of Research: Cosmic ray origins and high-energy particle acceleration by micro-quasars
Article Title: [Not explicitly provided in the content]
News Publication Date: 16-Nov-2025
Web References: http://dx.doi.org/10.1016/j.scib.2025.10.048
Image Credits: LHAASO Collaboration
Keywords
Cosmic rays; Micro-quasars; Black hole accretion; Ultra-high-energy gamma rays; Particle acceleration; PeV cosmic rays; LHAASO observatory; Astroparticle physics; Galactic cosmic ray sources
