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The constellation Cygnus, prominently featured in the northern summer sky and one of the most active and complex star-forming environments in the Milky Way, has long been a focal point for high-energy astrophysics. A central mystery in this region involves the "Cygnus Cocoon" - a source of ultra-high-energy gamma rays that may be a "PeVatron", a natural accelerator capable of pushing particles to the peta-electronvolt (1 PeV being the energy equivalent of a common housefly flying with about 2 m/s) scale. Such sources are sought to explain the origins of cosmic rays above the “knee”, an important feature in the spectrum of cosmic rays measured at Earth.
In a study published in Astronomy & Astrophysics, Härer et al. propose a compelling framework to explain this emission. Their 3D hydrodynamic simulations reveal that these ultra-high-energy gamma rays signatures likely originate in a 50,000-year-old supernova remnant. This remnant expanded within a “superbubble” - a large cavity inflated by the combined action of winds from numerous stars over several million years.
The team’s research provides a detailed look at how high-energy protons, accelerated at the outer blast wave of a supernova remnant, propagate through the complex environment of the Cygnus OB2 stellar association and its local medium. By modeling the specific 3D structure of the surrounding gas in the Cygnus region, the researchers found that a single supernova remnant agrees well with many features of the observed gamma-ray data.
The model suggests that the emission observed by modern ultra-high energy gamma-ray observatories is the result of a relic population of protons. These particles, accelerated in the early stages of the 50,000 years old supernova remnant are now interacting with the dense gas structures surrounding the Cygnus region, producing high energetic gamma ray emission in nuclear collisions. “Our study demonstrates that the spectral shape and spatial extent of the gamma rays detected by observatories like LHAASO can be replicated by a single supernova event, indicating that supernova remnants are among the Galaxy's most powerful particle accelerators”, explains Lucia Härer, the lead author on this study. Beyond explaining the Cygnus Cocoon, this work also integrates 3D fluid dynamics with particle transport theory, creating a new template for studying how energy is distributed across other high-energy regions of the Milky Way.
Original publication:
Deciphering the gamma-ray emission in the Cygnus region
L. Härer, T. Vieu, F. Schulze, C. J. K. Larkin, and B. Reville
A&A, Volume 703, November 2025. DOI: https://doi.org/10.1051/0004-6361/202555531
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