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.
Highlights
Star-forming regions are among the sources of high-energy (HE) photons in our galaxy
The region at which the winds interact reach stochastic saturation. It is characterized by the presence of strong turbulence produced by the interacting winds and magnetohydrodynamic instabilities
We have investigated a colliding-wind binary system as a possible site of high-energy and very high–energy cosmic ray production
Summary
Star-forming regions are among the sources of high-energy (HE) photons in our galaxy. Given the context described above, a natural improvement of the theoretical modeling of particle acceleration in CWBs would be the development of a full kinetic description of particles as subject to magnetized plasma fluctuations derived from a full MHD simulation of the shocks This is the main objective of the present work. We provide the first model to integrate a population of thermal hadronic particles, subject to the local distributions of plasma velocity, density, and magnetic fields derived from a full MHD numerical simulation. We assumed that the plasma velocity is in units of 1000 km/s, and for the magnetization level of the winds, we set the magnetic field anchored at 100 G Both the stellar surfaces to range from B* stars are assumed to maintain the same 10−5 to intensity of surface magnetic fields during the whole simulation time. In the integration of the particle trajectories, we set the absolute and relative tolerance to atol rtol 1e-10 and the initial time step dt 1e-6, if not specified otherwise
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