Mystery of 'knee' formation solved

Chinese and international scientists have identified powerful jets from black holes as the engines behind some of the highest-energy particles in the Milky Way — particles whose energies can be thousands of times greater than those produced by the world's most powerful man-made particle accelerators.

The finding helps explain a long-mysterious "knee" structure in the energy spectrum of cosmic rays, a sharp drop in the number of particles at higher energies. The structure was identified about 70 years ago, but the specific astrophysical sources responsible had remained unclear.

Using data from China's Large High Altitude Air Shower Observatory, or LHAASO, researchers detected ultra-high-energy gamma rays from five "microquasars" — compact systems in which a black hole pulls in material from a companion star and ejects near light-speed jets. Their studies were published in the journals National Science Review and Science Bulletin on Sunday.

The research was carried out by teams 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.

The LHAASO observations suggest that the cosmic rays producing these gamma rays may reach energies above 10 peta-electronvolts — surpassing the threshold of the "knee" region. For decades, scientists generally believed that known cosmic ray sources, such as supernova remnants, could not reach such extreme energies either observationally or theoretically.

To probe the "knee" more precisely, scientists need to measure the energy spectra of individual types of cosmic rays, such as protons, helium and heavier nuclei. But cosmic rays in this energy range are very rare, and the atmosphere scrambles their signatures, making it very difficult to distinguish one type from another.

By using LHAASO as a multi-parameter measurement facility, the team was able to collect a large sample of high-purity protons — the lightest and most abundant cosmic ray particles — with precision close to that of satellite-based detectors. This allowed them to map the proton energy spectrum more precisely than ever before.

Instead of showing a smooth curve as expected, the proton spectrum revealed a new, distinct "high-energy component". The result indicates that multiple kinds of cosmic accelerators exist in the Milky Way, each boosting particles to high energy with different limits. The "knee", scientists say, marks the maximum energy reached by the sources responsible for the newly identified component.

"This is a major step forward," said Cao Zhen, an academician of the Chinese Academy of Sciences and LHAASO's chief scientist. "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 that there are around a dozen such sources in the part of the galaxy monitored by LHAASO.

He said continued observations will be needed to identify more of these sources and to study the energy spectra of different cosmic-ray nuclei, which will help uncover how these particles are accelerated and how black hole systems work as accelerators. Understanding these natural accelerators, he added, could also help improve the design of future man-made particle accelerators.

LHAASO, designed, built and operated by Chinese scientists, is located in Daocheng county, Sichuan province. Since it began operating in July 2019, the facility has become a global leader in cosmic ray research because of its sensitivity to gamma ray sources and high-precision particle measurements. After receiving national approval in 2023, LHAASO has reported a series of discoveries that are reshaping scientific understanding of some of the most extreme processes in the universe.