The counterjet dominates the production of PeV photons from Cyg X-3

TL;DR

This paper explains the origin of PeV photons from Cyg X-3, highlighting the dominant role of the counterjet in hadronic interactions and contrasting it with GeV photon production in the approaching jet.

Contribution

It introduces a model where PeV photons are mainly produced by hadronic collisions in the counterjet, differing from the GeV emission in the approaching jet.

Findings

PeV photons originate mainly from the counterjet due to hadronic collisions.

Optical depths are low, making photon production proportional to column densities.

Phase differences in photon fluxes are explained by jet orientation and particle interactions.

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 and strongly magnetized region within the jet, but they then quickly advect downstream to 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 $\lesssim$1 for both types of interactions, implying that their rates are proportional to the column densities along the particle paths. Given the low viewing angle of Cyg X-3 ($i\approx26^\circ$--$28^\circ$), 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 the peak of the phase-folded PeV photon flux to be on the opposite side of the superior conjunction than that for 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.