Gamma rays surpassing 1 PeV observed for first time in Milky Way
Some cosmic rays have more than 10 Peta-Electronvolt energy, millions of times higher than the largest human-made particle accelerator, the Large Hadron Collider.
A team of Chinese researchers has found a giant bubble of ultra-high-energy gamma rays in the Cygnus region of the Milky Way, which reveals the origin of the most energetic cosmic rays ever detected.
Peta-Electronvolt cosmic rays
Cosmic rays are mysterious particles from outer space that bombard the Earth with incredible energies. Some have more than 10 Peta-Electronvolt (PeV, 1PeV=1015eV) energy, millions of times higher than the largest human-made particle accelerator, the Large Hadron Collider. But where do these cosmic rays come from, and how are they accelerated to such extreme energies?
This question has puzzled scientists for decades, especially since the discovery of a “knee” in the cosmic ray energy spectrum around 1 PeV. This means that the number of cosmic rays drops sharply above this energy, suggesting a limit to how much energy most sources in the Milky Way can provide. However, some cosmic rays still manage to surpass this limit and reach even higher energies, indicating that some super cosmic ray accelerators must lurk in the galaxy.
Now, a team of researchers from the Institute of High Energy Physics of the Chinese Academy of Sciences has found the first evidence of such a super cosmic ray accelerator, using the Large High Altitude Air Shower Observatory (LHAASO), a cutting-edge facility located at an altitude of 4,410 meters in Sichuan, China. LHAASO is designed to detect gamma rays, which are high-energy photons produced when cosmic rays interact with matter or radiation in space.
A huge bubble of gamma rays in the Cygnus region
The researchers reported in Science Bulletin that they discovered a huge bubble of gamma rays in the Cygnus star-forming region, which spans about 10 degrees in the sky, equivalent to 20 times the size of the full moon. Inside this bubble, they detected several gamma-ray photons with energies above 1 PeV, with the highest reaching 2.5 PeV. This is the first time gamma rays with such high energies have been observed in the Milky Way.
The gamma-ray bubble indicates a super cosmic ray accelerator inside it, which can boost protons to energies of up to 20 PeV and inject them into interstellar space. The researchers identified a massive star cluster called Cygnus OB2 as the most likely source of the super cosmic ray accelerator. Cygnus OB2 contains hundreds of young, hot, and massive stars that emit intense radiation and stellar winds, creating a turbulent environment that can accelerate particles to extreme energies.
The discovery of the super cosmic ray accelerator in the Cygnus region is a breakthrough for cosmic ray physics, as it sheds light on the origin of the highest-energy cosmic rays in the galaxy. It also shows that LHAASO is a powerful tool for exploring the high-energy universe and unveiling its mysteries. The researchers expect that LHAASO will find more super cosmic ray accelerators in the future and help solve the long-standing puzzle of the origin of cosmic rays.
Prof. Elena Amato, a renowned astrophysicist from the Italian National Institute for Astrophysical (INAF), highlighted the impact of the discovery on the origin of cosmic rays. She also commented that these results “not only impacts our understanding of diffuse emission, but has also very relevant consequences on our description of cosmic ray (CR) transport in the Galaxy.”
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