For decades, scientists have puzzled over a sharp bend in the cosmic ray energy spectrum, known as the “knee,” at around 3 PeV (peta-electron volts). Now, groundbreaking results from the Large High Altitude Air Shower Observatory (LHAASO) confirm that powerful particle accelerators within black hole systems—specifically, micro-quasars—are the likely source of this cosmic phenomenon. This discovery not only resolves a long-standing mystery but also reveals the critical role black holes play in shaping high-energy cosmic rays within our galaxy.
The Decades-Old Puzzle of the Cosmic Ray Knee
Cosmic rays, high-energy particles bombarding Earth from space, exhibit a spectrum where the number of particles decreases with increasing energy. However, at around 3 PeV, the spectrum takes a distinct “knee” shape—a sudden drop in particle count. The cause of this abrupt change remained unknown for nearly 70 years, with theories pointing to limitations in astrophysical acceleration processes.
LHAASO’s Breakthrough: Micro-Quasars as Cosmic Accelerators
Recent studies, published in National Science Review and Science Bulletin, demonstrate that micro-quasars—binary systems where a black hole accretes material from a companion star—are capable of accelerating particles to PeV energies. LHAASO systematically detected ultra-high-energy gamma rays from five micro-quasars: SS 433, V4641 Sgr, GRS 1915+105, MAXI J1820+070, and Cygnus X-1.
The ultra-high-energy radiation from SS 433 was found to overlap with a giant atomic cloud, indicating that high-energy protons are accelerated by the black hole and collide with surrounding matter. The proton energy in this system exceeded 1 PeV, with a total power output equivalent to four trillion hydrogen bombs detonating per second. V4641 Sgr exhibited gamma-ray energy reaching 0.8 PeV, making it another “super PeV particle accelerator.”
Why Supernova Remnants Aren’t Enough
Historically, supernova remnants were considered the primary source of cosmic rays. However, observational and theoretical studies have shown that they lack the energy to accelerate particles to PeV levels and beyond. LHAASO’s findings confirm that micro-quasars fill this gap, providing the necessary acceleration capabilities to produce the high-energy component of cosmic rays.
The Challenge of Measuring Cosmic Ray Spectra
Precisely measuring the energy spectra of cosmic rays, especially protons, is extremely difficult. The “knee” region is sparse, making detection akin to finding a needle in a haystack. Ground-based measurements are complicated by atmospheric interference, making it hard to distinguish protons from other nuclei. LHAASO overcame these challenges by leveraging its advanced observational equipment and developing multi-parameter measurement techniques.
Multiple Accelerators Shape the Cosmic Ray Landscape
LHAASO’s findings, combined with data from the space-borne AMS-02 and DArk Matter Particle Explorer (DAMPE) experiments, reveal a complex picture. The galaxy contains multiple accelerators, each with its own unique capabilities and energy range. The “knee” represents the acceleration limit of the sources responsible for generating the high-energy component, with micro-quasars playing a crucial role.
Connecting Black Holes to the Cosmic Ray Knee
LHAASO’s hybrid detector array design allows for the detection of cosmic ray sources through ultra-high-energy gamma rays, while enabling precise measurement of cosmic ray particles. This approach provides insights into the acceleration capabilities of sources at PeV energies and the spectral characteristics they contribute to cosmic rays. For the first time, the “knee” structure has been observationally connected to a specific type of astrophysical source—the black hole jet system.
A Chinese Breakthrough in Cosmic Ray Research
LHAASO, designed, constructed, and operated by Chinese scientists, has taken the lead in high-energy cosmic-ray research due to its sensitivity in both gamma-ray astronomy and cosmic ray precision measurement. Its series of discoveries have had a global impact, contributing to our knowledge of extreme physical processes in the universe.
In conclusion, LHAASO’s findings provide definitive evidence that black hole systems, specifically micro-quasars, are the primary source of high-energy cosmic rays responsible for the “knee” in the energy spectrum. This breakthrough not only resolves a decades-old mystery but also establishes a critical link between black holes and the extreme particle acceleration occurring in our galaxy
