3D Magnetohydrodynamic Models of Nonthermal Photon Emission in the Binary System γ² Velorum

Abstract

Recent reports claiming association of the massive star binary system γ 2 Velorum (WR 11) with a high-energy γ-ray source observed by Fermi-LAT contrast the so-far exclusive role of η Carinae as the hitherto only detected γ-ray emitter in the source class of particle-accelerating colliding-wind binary (CWB) systems. We offer support to this claim of association by providing dedicated model predictions for the nonthermal photon emission spectrum of γ 2 Velorum.We use three-dimensional magneto-hydrodynamic modeling (MHD) to investigate the structure and conditions of the wind-collision region (WCR) of γ 2 Velorum including the important effect of radiative braking in the stellar winds. A transport equation is then solved throughout the computational domain to study the propagation of relativistic electrons and protons. The resulting distributions of particles are subsequently used to compute nonthermal photon emission components.In agreement with observation in X-ray spectroscopy, our simulations yield a large shock-cone opening angle. We find the nonthermal γ-ray emission of γ 2 Velorum to be of hadronic origin owing to the strong radiation fields in the binary system which inhibit the acceleration of electrons to energies sufficiently high for efficient inverse Compton radiation. We also discuss the strong dependence of a hadronic γ-ray component on the energy-dependent diffusion used in the simulations. Of two mass-loss rates for the WR star found in literature, only the higher one is able to accommodate the observed γ-ray spectrum with reasonable values for important simulation parameters such as the injection ratio of high-energy particles within the WCR.

Hints of γ-ray orbital variability from γ2 Velorum

“…Therefore a physical relation between this particular binary and the HE signal is supported, potentially discarding other nearby non-thermal radio sources as the origin of the γ-rays (Benaglia 2016;Benaglia et al 2019). The system is studied and modelled by Reitberger et al (2017) using 3D MHD simulations. Although these authors do not use the exact values of the spectrum from Pshirkov (2016) owing to an erratum (see the deviation of the model with respect the data in Figure 5), they report similar flux levels.…”

“…The different properties of both systems can be connected to conditions in the WCR. According to Reitberger et al (2017), electron acceleration in γ 2 Velorum only reaches 10 MeV at the apex and 100 MeV in the outer wings of the WCR as a consquence of the strong radiation field. Therefore, while π 0 decay can be responsible for the observed spectrum (see Section 4.1), IC losses in the WCR prevent electrons from reaching higher energies in this binary and its leptonic component lies out of the Fermi-LAT detection capabilities.…”

“…Data fromPshirkov (2016) are shown in grey. Emission fromReitberger et al (2017) at apastron is shown in blue. The spectrum was obtained assuming Γ = 2.0 per each energy bin by default with fermipy (Γ = 2.39 provides fully compatible results).…”

Overview of non-transientγ-ray binaries and prospects for the Cherenkov Telescope Array

“…At a distance of d=340 pc, the high energy flux of F(0.1-100 GeV) = (2.7±0.5)× 10 −12 erg cm −2 s −1 corresponds to a luminosity of L=(3.7±0.7)× 10 31 erg s −1 . This is only a small fraction (∼ 10 −4 ) of the wind kinetic power dissipated in the colliding wind zone (Pshirkov 2016;Reitberger et al 2017).…”

“…The first 3 dimensional MHD simulations of the colliding wind region in γ 2 Vel, which took into account the generation of γ-ray emission via diffusive shock acceleration of protons and nuclei, and subsequent pion decay, is presented in Reitberger et al (2017). The Fermi-LAT data (Pshirkov 2016) can only be reproduced using a high WR mass-loss rate (Ṁ WR = 3×10 −5 M yr −1 ), but the observed hardening of the γ-ray spectrum at 10-100 GeV is not reproduced since the simulated spectrum has a cut-off at 100 GeV due to the accelerated protons reaching a maximum energy of ∼ 1 TeV.…”