Two Models for the Orbital Modulation of Gamma Rays in Cyg X-3

We model the currently available gamma-ray data on Cyg X-3 from the Fermi Large Area Telescope. Thanks to Cyg X-3's very strong gamma-ray activity during 2018-2021, the data quality has significantly improved. We study the strong orbital modulation of the gamma-rays observed at high gamma-ray fluxes. The modulation, as found earlier, is well modeled by anisotropic Compton scattering of the donor blackbody emission by relativistic electrons in a jet strongly misaligned with respect to the orbital axis. We confirm that this model fits well both the average gamma-ray modulation light curve and the spectrum. However, we find that if the jet were aligned with the spin axis of a rotating black hole, it would undergo geodetic precession with a period of similar to 50 yr. However, its presence is ruled out by both the gamma-ray and radio data. Therefore, we consider an alternative model in which the average jet direction is aligned, but it is bent outside the orbit owing to the thrust of the donor stellar wind, and thus precesses at the orbital period. The gamma-ray modulation then appears as a result of the variable Doppler boosting of synchrotron self-Compton jet emission. This model also fits the data well. However, the fitted bending angle is much larger than the theoretical one based on the binary and wind parameters as currently known. Thus, both models disagree with important aspects of our current theoretical understanding of the system. We discuss possible ways to find the correct model.