Determination of physical parameters of particle-accelerating colliding-wind binaries based on generalized partition considerations

Space Sciences, Technologies and Astrophysics Research (STAR) Institute, University of Liège, Quartier Agora, 19c, Allée du 6 Août, B5c, 4000 Sart Tilman, Belgium
e-mail: Michael.DeBecker@uliege.be

Received: 17 August 2018
Accepted: 7 October 2018

Abstract

The sub-set of massive binaries known to accelerate particles, the so-called category of particle-accelerating colliding-wind binaries (PACWBs), constitutes a valuable laboratory for investigating the non-thermal physics in stellar environments. In particular, their synchrotron emitter status allows us to derive some basic properties of their population of relativistic electrons. In this paper, considerations about energy partitions (not restricted to equipartition or minimum energy) are developed to derive relevant physical parameters of PACWBs in an appropriate assumption context. This approach was applied to three PACWBs with known stellar wind and orbital parameters, along with rather well-known radio properties. For long-period systems, a local magnetic field of the order of 0.1–10 mG was determined for a wide range of assumptions on partition parameters, while values of a few G are obtained for the shorter period system we investigated. Normalization parameters of the relativistic electron populations were also determined. Synchrotron self-absorption appears to be an unlikely turn-over process for long-period systems, while it may compete with free-free absorption for a shorter period object. Our results are discussed in the context of the energy budget of non-thermal processes in PACWBs, and prospects for high energy emission are also addressed. The sensitivity of this approach, applied for the first time to PACWBs, was also investigated through a critical discussion of the dependence of determined physical quantities on adopted and assumed parameters. Even though this method is certainly not adequate to reproduce the physics of PACWBs in detail, it offers the advantage of allowing for the determination of valuable average quantities provided a few fundamental parameters and measurements are known, without the need of any detailed hydrodynamic and radiative modelling. In the absence of any relevant measurement of non-thermal radiation in the high-energy domain, the method presented here constitutes the most straightforward and accessible approach to date to discuss physical parameters that are relevant for the non-thermal physics of PACWBs.