The Counterjet Dominates the Production of PeV Photons from Cyg X-3

Abstract We study the physical mechanisms underlying the production of orbitally modulated PeV photons from Cyg X-3, recently discovered by the LHAASO collaboration. Our key findings are as follows. Helium nuclei are accelerated in a compact, strongly magnetized region within the jet, but they then advect quickly downstream into regions with a weaker field, allowing them to diffuse out of the jet, where they produce pions in hadronic collisions with both the stellar photons and the stellar wind of the Wolf–Rayet donor. The optical depths across the binary are ≲1 for both types of interactions, implying that their rates are proportional to the column densities along the particle paths. Given the low viewing angle of Cyg X-3 ( i ≈ 26°–28°), most of the observed photons are produced by the relativistic hadrons accelerated in the counterjet (for which the column densities toward the observer are much longer than for the jet). This also explains why the phase-folded PeV photon flux peaks on the opposite side of superior conjunction from the (also orbitally modulated) GeV photons, which are produced by collisions of relativistic electrons with stellar photons in the optically thick regime. This then implies that the GeV emission is produced in the approaching jet.