Constraining the PeV Gamma-Ray Emission Zone of Cygnus X-3 with Contemporaneous GeV Timing and Spectral Observations

Cygnus X-3 has recently been established as a variable ultra-high-energy gamma-ray source with photons detected up to 3.7 PeV. Its PeV activity is temporally correlated with Fermi-LAT GeV flares and may also be orbitally modulated, suggesting that the emission is produced within or close to the binary system. We test whether the contemporaneous GeV emission zone can also host the acceleration of the parent protons responsible for the multi-PeV photons. We jointly model the contemporaneous Fermi-LAT spectrum and orbital light curve with a one-zone leptonic scenario dominated by anisotropic external inverse-Compton scattering. The fit places the GeV emission region at H ∼ 2.8 × 10 11 cm and constrains the magnetic field–size product to BH ≲ 10 13.3 G cm at the 3 σ level. This implies a maximum proton energy of only ∼0.3 PeV from the Hillas criterion, far below that required by the observed PeV emission. We therefore conclude that the GeV zone cannot be the main PeV acceleration site. Instead, the PeV emission should arise from a more compact inner region, requiring substantial magnetic-field dissipation between the PeV and GeV emitting zones.