Chinese observatory links black hole jets to cosmic ray acceleration, unveils mystery of 'knee' structure

What mechanism accelerates cosmic rays to the 3 peta-electron-volt (PeV) range — over 1,000 times the energy of the world's most powerful man-made particle accelerators — causing a sharp decline in the number of higher-energy cosmic rays and forming a knee-like shape in their energy spectrum?

Using data from China's Large High Altitude Air Shower Observatory (LHAASO), a team of Chinese and international scientists has identified powerful jets from black holes as the "engines" driving high-energy particles in the Milky Way. This marks a milestone in observationally connecting the "knee" structure to a specific type of astrophysical source since its discovery 70 years ago.

The studies, published in the journals National Science Review and Science Bulletin on Sunday, were conducted by researchers from the Institute of High Energy Physics of the Chinese Academy of Sciences, Nanjing University, the University of Science and Technology of China, and Italy's Sapienza University of Rome.

Black holes, among the most enigmatic objects in the universe, generate near light-speed jets when pulling in material from companion stars in binary systems, forming micro-quasars.

In this study, LHAASO systematically detected ultra-high-energy gamma rays from five micro-quasars for the first time. Results suggest that the primary cosmic rays generating these gamma rays may have energies exceeding 10 PeV — surpassing the energy threshold of the "knee" region. This finding addressed the long-standing issue that the previously recognized cosmic ray sources, such as supernova remnants, couldn't surpass this threshold observationally or theoretically.

To further understand the phenomenon, precise measurements of the energy spectra of various cosmic ray species and their respective "knee" regions are essential. However, due to the rarity of cosmic rays in the "knee" region and atmospheric interference, distinguishing protons from other nuclei was very challenging and once considered impossible.

Based on LHAASO's multiparameter measurement techniques, the research team obtained a large statistical sample of high-purity protons with precision comparable to that of satellite experiments. This enabled them to conduct precise measurements of the energy spectrum of protons, which are the lightest and most easily detectable and explainable nuclei.

This measurement revealed an energy spectrum structure that was entirely unexpected, clearly displaying a new "high-energy component" instead of a simple transition between power-law spectra. This finding suggested the existence of multiple accelerators within the Milky Way, each possessing its own unique acceleration capability and energy range. The "knee" represents the acceleration limit of the sources responsible for generating the high-energy component.

"This is a major step forward," said Cao Zhen, an academician of the Chinese Academy of Sciences and chief scientist of LHAASO. "For the first time globally, scientists have observed a type of source that can truly provide an explanation for the cosmic rays at the 'knee' region," Cao said, adding that there are estimated to be a dozen or so such sources.

Cao said further observation and studies are needed to detect these sources and explore the energy spectrum structure of different nuclei to advance the understanding of the mechanism of cosmic ray origins and the extreme physical processes of black hole systems. He added that the working principles of some artificial accelerators are similar to those of cosmic ray accelerations, and understanding these mechanisms has the potential to be applied in upgrading the equipment.

LHAASO, a ground-based cosmic ray observatory designed, constructed, and operated by Chinese scientists, is located at a high altitude in Daocheng county, Sichuan province. Construction commenced in 2016, and it has taken the lead in high-energy cosmic ray research due to its sensitivity in both gamma-ray astronomical exploration and cosmic ray precision measurement. Receiving national approval in 2023, LHAASO has achieved a series of discoveries with global impact, contributing to our understanding of the extreme physical processes in the universe.