Parameter constraints for high-energy models of colliding winds of massive stars: the case WR 147

TL;DR

This paper investigates how high-energy observations can constrain the orbital parameters of long-period massive binary systems, specifically WR 147, using inverse Compton models and recent gamma-ray data.

Contribution

It introduces a method to infer orbital inclination and geometry of WR 147 from high-energy gamma-ray observations, especially when traditional methods are ineffective.

Findings

High-energy gamma-ray data suggest WR 147 likely has a large inclination angle.

Inverse Compton scattering is sensitive to the system's line-of-sight angle.

Electron acceleration efficiency impacts the detectability of GeV photons.

Abstract

We explore the ability of high energy observations to constrain orbital parameters of long period massive binary systems by means of an inverse Compton model acting in colliding wind environments. This is particular relevant for (very) long period binaries where orbital parameters are often poorly known from conventional methods, as is the case e.g. for the Wolf-Rayet (WR) star binary system WR 147 where INTEGRAL and MAGIC upper limits on the high-energy emission have recently been presented. We conduct a parameter study of the set of free quantities describing the yet vaguely constrained geometry and respective effects on the non-thermal high-energy radiation from WR 147. The results are confronted with the recently obtained high-energy observations and with sensitivities of contemporaneous high-energy instruments like Fermi-LAT. For binaries with sufficient long periods, like WR 147, gamma-ray attenuation is unlikely to cause any distinctive features in the high-energy spectrum. This leaves the anisotropic inverse Compton scattering as the only process that reacts sensitively on the line-of-sight angle with respect to the orbital plane, and therefore allows the deduction of system parameters even from observations not covering a substantial part of the orbit. Provided that particle acceleration acts sufficiently effectively to allow the production of GeV photons through inverse Compton scattering, our analysis indicates a preference for WR 147 to possess a large inclination angle. Otherwise, for low inclination angles, electron acceleration is constrained to be less efficient as anticipated here.