Microquasar Cygnus X-3 as the PeVatron powering the Cygnus Bubble

Abstract

The recent discovery by the LHAASO collaboration of a variable ultra-high-energy (UHE; $E_\gamma \ge$ 100 TeV) $\gamma$-ray source associated with the microquasar Cygnus X-3, with a spectrum extending to several PeV, provides compelling evidence for a hadronic super-PeVatron operating within the binary system. Inside the binary, the accelerated protons lose only a small fraction of their energy; upon escaping into the interstellar medium, they propagate diffusively to form a vast gamma-ray ``halo" structure extended to hundreds of parsecs. We argue that this halo has already been detected and corresponds to the Cygnus Bubble, an extended UHE $\gamma$-ray source reported by the LHAASO collaboration -- which possesses an angular extension of $\approx 6^{\circ}$ and an energy spectrum reaching 1 PeV. While the Cygnus Bubble is generally attributed to the star-forming region Cygnus X (specifically the Cygnus OB2 association at 1.4 kpc), we demonstrate that an association with Cygnus X-3 is physically more natural at energies above 400 TeV. This is supported by the cosmic-ray radial distribution, derived from the $\gamma$-ray and gas distributions, which points to continuous injection from a point-like source. The energetic requirements of the central accelerator are reasonably affordable and feasible. This reassignment identifies the Cygnus Bubble as a member of the recently discovered population of microquasar UHE $\gamma$-ray halos.