This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Astronomers have proposed a new explanation for the mysterious Cygnus Bubble, a vast cloud of ultra-high-energy gamma rays stretching thousands of light-years across the sky. While the bubble has generally been linked to the Cygnus X star-forming region, the new study argues that a microquasar more naturally explains its highest-energy emission.
The results were published in a paper in The Astrophysical Journal Letters on July 21. For years, astronomers have puzzled over the sources of the highest-energy cosmic rays in our galaxy—particles accelerated to a quadrillion electron volts, or a petaelectronvolt (PeV; that's 15 zeros after 1). These sources are called Galactic PeVatrons, and they are notoriously hard to pin down.
In principle, all kinds of astrophysical objects violent enough to accelerate particles to extreme energies could be PeVatrons: supernova remnants, pulsar wind nebulae, star clusters or binary star systems. The accelerated particles travel outward and light up with ultra-high-energy gamma rays wherever they eventually slam into ambient gas, sometimes hundreds to thousands of light-years from their true source. One of the most striking gamma-ray structures in the sky, the "Cygnus Bubble," has long been attributed to a nearby cluster of massive, young stars in the Cygnus X star-forming region, some 4,600 light-years away.
In this study, a team of astronomers led by Zhaodong Shi of the University of Science and Technology of China investigates whether a microquasar happens to be the PeVatron that created the Cygnus Bubble. A microquasar is a type of binary system consisting of a compact object (a black hole or neutron star) orbiting a normal star, where the compact object pulls in material from its companion and launches powerful jets or winds. The microquasar in question, Cygnus X-3, is much farther away than the traditional suspect, at roughly 31,600 light-years away.
The system's orbital period is 4.8 hours. The idea gains extra weight from a separate recent LHAASO finding: Cygnus X-3 was confirmed as the Milky Way's first "super-PeVatron," accelerating particles to at least 30 PeV in sync with its 4.8-hour orbit—direct evidence that extreme acceleration is happening within the binary itself. To test their idea, the researchers built a model simulating Cygnus X-3 as a continuous source, spraying high-energy protons into space for hundreds of thousands of years.
As these particles slowly spread through the surrounding gas, they occasionally collide with gas atoms, producing the gamma rays detected on Earth. When the team tuned the model to reasonable physical values, it successfully reproduced both the total brightness of the Cygnus Bubble and the pattern of how that brightness fades outward from the center. They found that Cygnus X-3 would need to funnel only 1% to 3% of its power into particle acceleration, and the particle-spreading rate needed to match the data lined up well with independent theoretical predictions.
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