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 The Astrophysical Journal Letters.
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). These sources are called Galactic PeVatrons, and they are notoriously hard to pin down. In principle, any astrophysical object violent enough to accelerate particles to extreme energies could be a PeVatron: 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. That makes tracing a bubble back to its origin difficult.
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. The new study instead points to a microquasar — a compact object such as a black hole or neutron star that feeds on a companion star and launches powerful jets — as the more plausible engine of the bubble's most energetic radiation.
If confirmed, the finding would recast how astronomers map the Milky Way's cosmic-ray factories, adding compact binary systems to the short list of known Galactic PeVatrons and sharpening the hunt for the true sources behind the galaxy's most extreme particles.
