The recent detection by LHAASO of a variable ultra-high-energy gamma-ray source (≥100 TeV) from the microquasar Cygnus X-3, with a spectrum extending to several PeV, indicates the operation of a hadronic super-PeVatron within the binary system. Accelerated protons lose only a small fraction of their energy inside the system; once they escape, they propagate diffusely, forming an extended gamma-ray halo hundreds of parsecs across. It is shown that this halo matches the previously discovered LHAASO object — the Cygnus Bubble — with an angular size of ≈6° and a spectrum up to 1 PeV. Although the bubble is traditionally linked to the Cygnus X star-forming region (the OB2 association at 1.4 kpc), this work demonstrates that above 400 TeV, an association with Cygnus X-3 is physically more natural. Analysis of the cosmic-ray radial profile, reconstructed from gamma-ray and gas data, points to continuous injection from a point-like source. The energetic requirements for the accelerator are well within reach. This identification places the Cygnus Bubble among the recently discovered population of ultra-high-energy microquasar gamma-ray halos.
The hunt for galactic cosmic rays of ultra-high energies is one of the central quests of astrophysics. Particles with energies up to several petaelectronvolts (PeV) are born in extreme objects called PeVatrons, but their precise locations remained elusive for a long time. Recently, the LHAASO observatory discovered dozens of extended gamma-ray sources that may be the footprints of such accelerators. Prominent among them is the Cygnus Bubble—a gigantic structure about 6 degrees across, previously attributed to a star-forming region. However, new research puts forward a bold hypothesis: behind the bubble lies the microquasar Cygnus X-3, a system where a compact object surrounded by an accretion disk launches powerful jets.
The researchers built a model of proton diffusion injected by Cygnus X-3 into the surrounding interstellar medium. The transport equation was solved analytically and through numerical simulations for different assumptions about plasma turbulence (Kolmogorov and Iroshnikov-Kraichnan spectra) and gas distribution. Energy losses from pion production in collisions with gas protons were accounted for, as well as the vertical inhomogeneity of the interstellar medium away from the Galactic plane. Then, using spectroscopic data from LHAASO, the gamma-ray profile was reconstructed and compared with predictions.
It turned out that with a diffusion coefficient D ≈ 3×10²⁹ cm²/s at 1 PeV and an injection duration of about 100,000 years, the model beautifully reproduces both the total flux and the radial distribution of gamma rays above 400 TeV. The key result: the power required to accelerate protons is only 0.7–3.2% of the jet kinetic power (~5×10³⁹ erg/s), which is quite realistic. Moreover, the brightness profile points to a point source continuously replenishing the relativistic particle population, not a distributed star-forming region. Thus, the Cygnus Bubble at energies above 400 TeV turns out to be not a cocoon of stellar winds, but a giant "halo" around the microquasar.
This discovery rewrites the scenario of cosmic ray generation in the Galaxy. Microquasars, previously considered supporting actors, step into the spotlight as efficient PeVatrons. Crucially, we see both the accelerator itself (Cygnus X-3) and its long-term afterglow—a gamma-ray halo built up over hundreds of thousands of years. This provides a unique opportunity to study the particle injection history and the properties of interstellar turbulence.
Future telescopes, such as CTA and LACT, with angular resolution better than 0.05°, will isolate the compact core of Cygnus X-3 and map in detail the energy dependence of the halo size. This will become a classic test for a discrete injector: with increasing energy, the gamma-ray emission should shrink toward the source. Moreover, detailed spectrum measurements will refine the mechanism of Fermi acceleration at shock fronts in the jets.
This work touches on cosmic-ray physics, interstellar medium dynamics, and accretion theory; it also spurs the search for analogous objects—'microquasar halos'—among other extended LHAASO sources.
High-resolution multi-wavelength observations (radio, X-ray, gamma) are needed to confirm the connection of compact Cygnus X-3 to the bubble, as well as simulations accounting for the back-reaction of cosmic rays on the gas (the 'cavity clearing' effect).
This research directly addresses the origin of ultra-high-energy galactic cosmic rays and the nature of PeVatrons. It also raises the question of diffusion suppression mechanisms near powerful accelerators—a necessary condition for forming the observed halos.
🎯 The energy of a single proton at 10 PeV is comparable to a flying mosquito's kinetic energy, but squeezed into a subatomic volume. And Cygnus X-3 itself is so bright in radio that it's sometimes called a 'mini-quasar'—like a real quasar, just inside our own Galaxy.
🎬 The idea that microquasars could serve as natural accelerators echoes the concept of 'wormholes' or sci-fi engines using relativistic jets for interstellar travel—only here the jets don't propel a ship, but spawn particles that pierce through the Galaxy.