3D simulations of wind-jet interaction in massive X-ray binaries

High-mass microquasars may produce jets that will strongly interact with surrounding stellar winds on binary system spatial scales. We study the dynamics of the collision between a mildly relativistic hydrodynamical jet of supersonic nature and the wind of an OB star. We performed numerical 3D simulations of jets that cross the stellar wind with the code Ratpenat. The jet head generates a strong shock in the wind, and strong recollimation shocks occur due to the initial overpressure of the jet with its environment. These shocks can accelerate particles up to TeV energies and produce gamma-rays. The recollimation shock also strengthens jet asymmetric Kelvin-Helmholtz instabilities produced in the wind/jet contact discontinuity. This can lead to jet disruption even for jet powers of several times $10^{36}$ erg s$^{-1}$. High-mass microquasar jets likely suffer a strong recollimation shock that can be a site of particle acceleration up to very high energies, but also eventually lead to the disruption of the jet.

💡 Research Summary

This paper investigates the dynamical interaction between a mildly relativistic, supersonic hydrodynamic jet and the stellar wind of an OB companion in high‑mass microquasars, using fully three‑dimensional numerical simulations performed with the Ratpenat code. The authors motivate the study by noting that many high‑mass X‑ray binaries display non‑thermal radio, X‑ray, and very‑high‑energy (VHE) gamma‑ray emission, which could be produced where the jet collides with the dense wind on binary‑scale distances (∼10¹² cm).

The simulation setup models a jet with a bulk velocity of ≈0.3 c, Mach number ≈5, and an initial over‑pressure relative to the ambient wind (pressure ratio ≈10). Jet powers are explored in the range 10³⁶–10³⁷ erg s⁻¹, consistent with estimates for known microquasars. The companion wind is represented by a typical OB‑type mass‑loss rate of 10⁻⁶ M⊙ yr⁻¹ and a terminal speed of 2 × 10⁸ cm s⁻¹. An adaptive mesh refines the jet radius down to ~1/20 of its initial size, allowing the code to resolve thin shock fronts and shear layers.

Results show that the jet head drives a strong bow shock into the wind, compressing the wind material and forming a high‑pressure contact discontinuity. Because the jet is initially over‑pressured, a recollimation shock forms a few jet radii downstream, producing a compact region of enhanced density and pressure. This recollimation shock is a natural site for efficient particle acceleration: the shock compression ratio and the associated magnetic field amplification can generate electric fields capable of accelerating electrons and protons up to TeV energies.

Crucially, the recollimation shock also amplifies Kelvin‑Helmholtz (KH) instabilities that arise at the wind‑jet interface. The simulations demonstrate that even for jet powers of several × 10³⁶ erg s⁻¹, the KH modes grow non‑linearly, distort the jet cross‑section, and eventually lead to partial jet disruption. The jet remains globally directed but its outer layers become turbulent and mixed with wind material, a process that could explain observed jet bending and irregular radio morphologies in systems such as Cyg X‑1 and LS 5039.

The authors discuss the radiative implications of these findings. Particles accelerated at the recollimation shock can produce synchrotron emission in the radio to X‑ray bands and inverse‑Compton or hadronic gamma‑ray emission in the GeV–TeV range. The variability of the shock location, driven by wind inhomogeneities and KH turbulence, naturally yields time‑dependent high‑energy output, consistent with the rapid gamma‑ray flares reported for several microquasars.

In the discussion, the paper compares the simulation outcomes with observational constraints, emphasizing that a strong recollimation shock is likely a generic feature of high‑mass microquasar jets. The authors acknowledge limitations: the simulations are purely hydrodynamic (magnetic fields are not evolved self‑consistently) and assume a smooth, isotropic wind. They propose future work incorporating magnetohydrodynamics (MHD) and clumpy wind structures to assess their impact on shock formation, particle acceleration efficiency, and jet survivability.

In conclusion, the study provides a comprehensive three‑dimensional picture of jet–wind interaction in massive X‑ray binaries. It identifies the recollimation shock as a dual agent: a potent accelerator of particles to very high energies and a catalyst for Kelvin‑Helmholtz instabilities that can ultimately disrupt the jet. These results bridge the gap between theoretical jet dynamics and the high‑energy phenomenology observed in high‑mass microquasars, offering a robust framework for interpreting current and future multi‑wavelength observations.