Colliding-Wind Binaries as a Source of TeV Cosmic Rays

Abstract

In addition to gamma-ray binaries which contain a compact object, high-energy and very high–energy gamma rays have also been detected from colliding-wind binaries. The collision of the winds produces two strong shock fronts, one for each wind, both surrounding a shock region of compressed and heated plasma, where particles are accelerated to very high energies. Magnetic field is also amplified in the shocked region on which the acceleration of particles greatly depends. In this work, we performed full three-dimensional magnetohydrodynamic simulations of colliding winds coupled to a code that evolves the kinematics of passive charged test particles subject to the plasma fluctuations. After the run of a large ensemble of test particles with initial thermal distributions, we show that such shocks produce a nonthermal population (nearly 1% of total particles) of few tens of GeVs up to few TeVs, depending on the initial magnetization level of the stellar winds. We were able to determine the loci of fastest acceleration, in the range of MeV/s to GeV/s, to be related to the turbulent plasma with amplified magnetic field of the shock. These results show that colliding-wind binaries are indeed able to produce a significant population of high-energy particles, in relatively short timescales, compared to the dynamical and diffusion timescales.

Limits on the non-thermal emission of the WR–WR system Apep

“…Although we note that the limited η B values do not reach equipartition, the derived magnetic field limit (0.27 G B 0.13 G) is already at the high end of values expected within the WCR in generic CWBs (see e.g., Kissmann et al 2016;Kowal & Falceta-Gonçalves 2021). Relevant effects such as magnetic amplification of the initial magnetic field B 0 could also play an important role for high Alfvénic Mach numbers (B 2 tot /B 2 0 ∝ M Alfven , where B tot is the total magnetic field, see e.g., Caprioli & Spitkovsky 2014), which might exacerbate synchrotron particle losses -precluding γ-ray detection while still boosting bright radio emission.…”

The 2020 periastron of Eta Carinae at high-energies

“…Those strong winds will interact, forming a two-sided bow-shocked wind collision region (WCR) whose conditions change along the orbit [2]. Particle acceleration can occur there via diffusive shock acceleration (DSA), with cosmic rays eventually reaching up to TeV energies and leading to high-energy radiation [3][4][5].…”