In a groundbreaking achievement, the Large High Altitude Air Shower Observatory (LHAASO) has unveiled the discovery of a giant ultra-high-energy gamma-ray bubble in the Cygnus star-forming region, marking a significant milestone in the quest to understand cosmic rays. For the first time, scientists have traced cosmic rays carrying more than 10 Peta-Electronvolts (PeV) back to a specific source, a finding that could reshape our understanding of high-energy astrophysics.
The LHAASO Collaboration, led by Prof. Zhen Cao from the Chinese Academy of Sciences, reported this discovery in a cover article published in the Science Bulletin. The paper also features contributions from co-authors Dr. Chuandong Gao, Dr. Cong Li, Prof. Ruoyu Liu, and Prof. Ruizhi Yang. The discovery addresses one of the most anticipated questions in astrophysics: where do these ultra-high-energy cosmic rays originate?
Cosmic rays are primarily composed of protons and are charged particles that travel through space at impressive velocities. Over the decades, measurements of cosmic rays have painted a complex picture of their origins, with a notable feature known as the "knee" of the energy spectrum. This phenomenon, which occurs around 1 PeV, indicates a sudden change in the abundance of cosmic rays as a function of energy. Cosmic rays below this energy are believed to originate from sources within our Milky Way galaxy, while the origins of those in the knee region have remained largely a mystery.
The discovery of the gamma-ray bubble, which includes photons with energies surpassing 1 PeV and peaking at 2.5 PeV, suggests the presence of a "super cosmic ray accelerator" within this structure. This accelerator has the capacity to propel high-energy cosmic ray particles up to 20 PeV, injecting them into the vastness of interstellar space. As these cosmic rays interact with interstellar gas, they produce gamma rays, which are observed by LHAASO.
The association of this bubble with the massive star cluster Cygnus OB2, located near its core, is particularly intriguing. This group of young and hot massive stars plays a crucial role in the ongoing particle acceleration process. O-type and B-type stars, which reside within Cygnus OB2, emit radiation luminosities that are hundreds to millions of times greater than our Sun. Consequently, the immense radiation pressure of these stars is potent enough to fracture their surface material, creating dynamic stellar winds — some of the fastest in the universe, reaching thousands of kilometers per second.
It is this interaction between stellar winds and the surrounding interstellar medium, combined with the collision of those winds, that constructs optimal conditions for accelerating cosmic particles. This discovery thereby not only identifies the first known super cosmic ray accelerator but also paves the way for future discoveries as LHAASO continues its observational work.
Post-research analyses indicate a significant increase in cosmic ray density in the vicinity surrounding the bubble, exceeding the Galactic average. Such density anomalies may help explain previous observations of diffuse gamma-ray emissions from the Galactic Plane, which LHAASO had detected. Prof. Elena Amato, an esteemed astrophysicist from the Italian National Institute for Astrophysics, emphasized the transformative nature of this discovery. She stated that it “not only impacts our understanding of diffuse emission, but has also very relevant consequences on our description of cosmic ray transport in the Galaxy.”
Positioned at an altitude of 4410 meters on Mount Haizi in Daocheng County, Sichuan Province, China, LHAASO encompasses a composite array of detection technologies. This includes the world's most sensitive ultra-high-energy gamma-ray detector. Constructed from an expansive one-square-kilometer ground array featuring 5216 electromagnetic particle detectors and 1188 muon detectors, coupled with a 78,000-square-meter array of water Cherenkov detectors and 18 wide-angle Cherenkov telescopes, LHAASO is designed for high-performance operations.
Since its completion in July 2021, LHAASO has aimed for not only high-quality observations but also a cooperative framework for sharing data and collaborating internationally. Currently, 280 members from about 32 astrophysics research institutions engage in this collective scientific initiative. Through this model, the facility fosters an environment of transparency and cooperation, allowing researchers around the globe to benefit from its rich findings.
The implications of LHAASO's discoveries are monumental, promising to unveil insights into the origins of cosmic rays and their transport throughout the Galaxy. As researchers continue to study the Cygnus bubble, it is expected that more super cosmic ray accelerators will be identified, further elucidating this cosmic phenomenon.
With the scientific community eager for answers, the LHAASO discoveries stand as a beacon of progress. As Prof. Zhen Cao and his colleagues continue their explorations, the ultimate objective is clear: unravel the mysteries of cosmic ray origins and redefine our understanding of the mechanisms that govern the majestic cosmos.
