Aerial photo of LHAASO taken in 2022. Photo: Courtesy of the Institute of High Energy Physics

The Large High Altitude Air Shower Observatory (LHAASO) has discovered a giant ultra-high-energy gamma-ray bubble structure in the Cygnus star-formation region. This is the first time in human history that the origin of cosmic rays with energy higher than 10 Peta-Electronvolt (PeV) has been discovered, the Global Times learned from the Institute of High Energy Physics of the Chinese Academy of Sciences (CAS) on Monday.

Cosmic rays are charged particles from outer space, mainly composed of protons. The origin of cosmic rays is one of the most important frontier issues in modern astrophysics. Measurements of cosmic rays in past decades have revealed a break around 1 PeV in the energy spectrum, which is called the "knee" of the cosmic ray energy spectrum due to its shape resembling a knee joint. 

Scientists believe that cosmic rays with energy lower than the "knee" originate from astrophysical objects within the Milky Way, and the existence of the "knee" also indicates that the energy limit for accelerating protons from most of the cosmic ray sources in the Milky Way is around a few PeV. However, the origin of cosmic rays in the "knee" region is still an unsolved mystery and one of the most intriguing topics in cosmic ray research in recent years.

LHAASO has discovered a giant ultra-high-energy gamma-ray bubble structure in the Cygnus star-formation region, with multiple photons exceeding 1 PeV inside the structure, with the highest energy reaching 2.5 PeV, indicating the presence of a super cosmic ray accelerator inside the bubble. This super cosmic ray accelerator continuously accelerates high-energy cosmic ray particles with energies of up to 20 PeV and injects them into interstellar space, the Institute of High Energy Physics told the Global Times on Monday.

Through research on the interior of this bubble-like structure, scientists believe that there is a cosmic ray accelerator inside, which is the origin of cosmic rays, and this origin continuously produces cosmic ray particles with energies exceeding 10 Peta electron volts. Through their studies, scientists infer that the massive star cluster near the center of this bubble-like structure may be the source of the cosmic rays they receive, and they refer to it as the "O-star association" and "B-star association." 

This is the first super cosmic ray accelerator identified as of now, according to the Institute. With increasing observation time, LHAASO is expected to detect more super cosmic ray accelerators, and hopefully solve the mystery of the origin of cosmic rays in the Milky Way.

LHAASO's observation has also indicated that the super cosmic ray accelerator inside the bubble significantly increases the cosmic ray density in the surrounding interstellar space, far exceeding the average level of cosmic rays in the Milky Way. The spatial extension of the density excess even exceeds the observed range of bubbles. This provides a possible explanation for the excess of diffuse gamma-ray emission from the Galactic Plane previously detected by LHAASO.

Professor Elena Amato, a renowned astrophysicist from the Italian National Institute for Astrophysical, highlighted the impact of the discovery on the origin of Cosmic Rays in general. She also commented that these results "not only impact our understanding of diffuse emission, but they also have very relevant consequences on our description of cosmic ray (CR) transport in the Galaxy."

LHAASO is a national key scientific and technological infrastructure focusing on cosmic ray research, located at an altitude of 4410 meters on Mount Haizi in Daocheng county, Southwest China's Sichuan Province. It was completed in July 2021, and afterward it began high-quality and stable operation and it is the most sensitive ultra-high-energy gamma-ray detection device in the world.

Global Times