Astrophysicists have proposed that the giant gamma-ray bubble in Cygnus (angular size ~6°, energy up to 1 PeV) is not a by-product of a star-forming region, but a 'halo' around the microquasar Cygnus X-3. The previously detected variable ultra-high-energy source (E>100 TeV) from this binary system points to a natural proton accelerator — a super-PeVatron. Once they escape the system, protons scatter through the interstellar medium, creating an extended glow, much like a lighthouse beam cutting through a foggy harbor. This perspective explains the observed cosmic-ray distribution and places the bubble within a new class of objects — microquasar gamma-ray halos.
Deep in the constellation Cygnus beats a cosmic forge — the microquasar Cygnus X-3. You can't see it in optical light, but it forges particles in relativistic jets, hammering them against the anvil of its accretion disk. Protons accelerated to energies of tens of PeV — these are cosmic rays — burst into space and, like white-hot sparks from the forge, fly outward, filling the surroundings with a ghostly gamma-ray glow. We used to think this extended Cygnus Bubble was the aftermath of furious star formation, but new data force us to reconsider: we're not looking at a bonfire of many stars, but at a single hammer of incredible power.
To model how particles spread from the source, scientists used numerical simulations of diffusion in the interstellar medium. Around Cygnus X-3, the plasma is inhomogeneous, threaded with magnetic fields, and the escaping protons wander through this labyrinth. It turned out that at an energy of about 1 PeV, the diffusion coefficient is just D ≈ 3×10²⁹ cm²/s — the particles don't escape instantly; they linger, building up a gamma-ray halo accumulated over hundreds of thousands of years. This "slow" escape explains the emission profile reconstructed from LHAASO's spectroscopic data.
This turns the microquasar into a textbook PeVatron — a particle accelerator that reaches the PeV energy frontier. And most importantly, we see both the hammer itself and its long-lasting imprint in gamma-ray light. Such "microquasar halos" could be hiding among other extended sources, meaning the Galaxy is filled with these invisible forges. Future telescopes — CTA, LACT — with their sharp eyes will allow us to separate the compact core from the bubble and watch how, as energy increases, the gamma-ray halo shrinks toward the source. This will be the decisive test of the discrete injector scenario and will let us study in detail the mechanism of Fermi acceleration at shock waves in jets.
Thus, microquasars — once considered cosmic small fry — are taking center stage in the drama of the origin of the most energetic cosmic rays. Perhaps such "hearts" beat at the core of many enigmatic gamma-ray structures, and by learning to read their glow, we will be able to compile a true map of the Galaxy's invisible accelerators.
🎯 The energy of a single 10 PeV proton is comparable to that of a flying mosquito, but squeezed into a subatomic volume. And Cygnus X-3 itself is so bright in radio waves that it's sometimes called a "mini-quasar" — like a real quasar, only in our own Galaxy.
🎬 The idea that microquasars can serve as natural accelerators echoes the concept of "wormholes" or sci-fi engines that use relativistic jets for interstellar travel — except here the jets aren't carrying a ship; they're birthing particles that pierce the Galaxy.