Particle acceleration and non-thermal emission in colliding-wind binary systems

Abstract

We present a model for the creation of non-thermal particles via diffusive shock acceleration in a colliding-wind binary. Our model accounts for the oblique nature of the global shocks bounding the wind–wind collision region and the finite velocity of the scattering centres to the gas. It also includes magnetic field amplification by the cosmic ray induced streaming instability and the dynamical back reaction of the amplified field. We assume that the injection of the ions and electrons is independent of the shock obliquity and that the scattering centres move relative to the fluid at the Alfvén velocity (resulting in steeper non-thermal particle distributions). We find that the Mach number, Alfvénic Mach number, and transverse field strength vary strongly along and between the shocks, resulting in significant and non-linear variations in the particle acceleration efficiency and shock nature (turbulent versus non-turbulent). We find much reduced compression ratios at the oblique shocks in most of our models compared to our earlier work, though total gas compression ratios that exceed 20 can still be obtained in certain situations. We also investigate the dependence of the non-thermal emission on the stellar separation and determine when emission from secondary electrons becomes important. We finish by applying our model to WR 146, one of the brightest colliding wind binaries in the radio band. We are able to match the observed radio emission and find that roughly 30 per cent of the wind power at the shocks is channelled into non-thermal particles.

Radio emission from colliding outflows in high-mass X-ray binaries with strongly magnetized neutron stars

“…Detailed radiative models for high-energy emission from colliding wind binaries show that the GeV gamma-ray luminosity from inverse Compton scattering can be up to 10 2 − 10 4 times higher than the GHz radio luminosity, depending on the binary separation distance and the orbital phase (e.g. Pittard et al 2021). Similar ratios can be expected in our scenario, as it bears many similarities with the shock-wind models for massive binaries.…”

“…The inverse Compton scattered emission can extend and actually peak (in νLν units) in the gamma-ray band, as shown for colliding wind binary systems (e.g. Reimer et al 2006;Werner et al 2013;Pittard et al 2021). Even though many of these systems have been observed in radio wavelengths (for a catalog, see De Becker & Raucq 2013), the predicted non-thermal high-energy emission in X-rays and gamma-rays remains elusive.…”

“…Strong shocks formed by the colliding stellar winds in massive binary systems have been discussed as promising sites of particle acceleration and non-thermal radiation (e.g. Eichler & Usov 1993;Dougherty et al 2003;Pittard et al 2021). In our toy model, we assume that relativistic electrons are produced by first-order Fermi acceleration at the shocks of the colliding stellar wind and NS outflow, and compute the synchrotron emission produced in the region of the shocked stellar wind 3 .…”

“…For larger angles the approximation for a head-on collision of the winds at each point begins to break down (e.g. Pittard et al 2021). Moreover, we calculate the thermal Brehmsstrahlung of the Be wind as (Wright & Barlow 1975)…”

“…A more accurate calculation would require a multizone radiation model that follows the particle distribution along the streamlines of the post-shock fluids, taking into account radiative and adiabatic energy losses (e.g. del Palacio et al 2016;de la Cita et al 2017;van der Merwe et al 2020;Pittard et al 2021). Such multi-zone models can also account for potential changes in the particle injection spectrum (e.g.…”