Two models for the orbital modulation of $\gamma$-rays in Cyg X-3

TL;DR

This paper models gamma-ray orbital modulation in Cyg X-3, comparing anisotropic Compton scattering and jet precession models, revealing discrepancies with current theories and suggesting alternative explanations for observed data.

Contribution

It introduces and compares two models for gamma-ray modulation in Cyg X-3, highlighting their strengths and limitations in explaining observational data.

Findings

Both models fit the gamma-ray modulation data well.

The precession model is inconsistent with radio and gamma-ray data.

The bending angle required by the jet model exceeds theoretical expectations.

Abstract

We model the currently available $\gamma$-ray data from the Fermi Large Area Telescope on Cyg X-3. Thanks to its 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 during 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 is aligned with the spin axis of a rotating black hole, it would undergo geodetic precession with the period of $\sim$50 years. 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 jet is aligned, but it is bent to outside the orbit owing to the thrust of the donor stellar wind, and thus precesses at the orbital period. The $\gamma$-ray modulation appears then owing to the variable Doppler boosting of synchrotron self-Compton jet emission. The model also fits well the data. 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.