The non-thermal emission from the colliding-wind binary Apep

Abstract

The recently discovered massive binary system Apep is the most powerful synchrotron emitter among the known Galactic colliding-wind binaries. This makes this particular system of great interest to investigate stellar winds and the non-thermal processes associated with their shocks. This source was detected at various radio bands, and in addition the wind-collision region was resolved by means of very-long baseline interferometric observations. We use a non-thermal emission model for colliding-wind binaries to derive physical properties of this system. The observed morphology in the resolved maps allows us to estimate the system projection angle on the sky to be $\psi \approx 85^\circ$. The observed radio flux densities also allow us to characterise both the intrinsic synchrotron spectrum of the source and its modifications due to free--free absorption in the stellar winds at low frequencies; from this we derive mass-loss rates of the stars of $\dot{M}_\mathrm{WN} \approx 4\times10^{-5}$ $\mathrm{M}_\odot$ yr$^{-1}$ and $\dot{M}_\mathrm{WC} \approx 2.9\times10^{-5}$ $\mathrm{M}_\odot$ yr$^{-1}$. Finally, the broadband spectral energy distribution is calculated for different combinations of the remaining free parameters, namely the intensity of the magnetic field and the injected power in non-thermal particles. We show that the degeneracy of these two parameters can be solved with observations in the high-energy domain, most likely in the hard X-rays but also possibly in $\gamma$-rays under favourable conditions.