Thermal emission from bow shocks. III. Variable diffuse X-ray emission from stellar-wind bow shocks driven by dynamical instabilities

TL;DR

This study uses high-resolution 2D and 3D simulations to analyze how magnetic fields and instabilities affect X-ray emission from stellar wind bow shocks, revealing the importance of resolving Kelvin-Helmholtz instabilities for accurate predictions.

Contribution

It demonstrates the necessity of high spatial resolution in simulations to accurately predict X-ray emission and the effects of magnetic fields on bow shock morphology and stability.

Findings

Resolving Kelvin-Helmholtz instability is crucial for convergence.

X-ray emission varies by an order of magnitude over time.

Magnetic field orientation influences instability development and emission maps.

Abstract

X-ray emission from wind-driven bow shocks is both difficult to measure and predict, but may give important insights into the energy budget of the hot phase of the ISM by quantifying mixing at the interface between hot and warm gas phases. We investigate the effect of magnetic fields and numerical resolution on predicted X-ray emission and other observable properties of bow shocks, to study convergence properties and assess robustness of predicted observables from simulations. A suite of 2D and 3D HD and MHD simulations of bow shocks were run and analysed to generate synthetic emission maps and light curves in X-ray and infrared emission. Resolving the Kelvin-Helmholtz (KH) instability at the wind-ISM contact discontinuity is crucial for obtaining converged results and for predicting X-ray emission and the properties of the hot shocked wind. When sufficient spatial resolution is used, we measure time variation of X-ray emission of at least an order of magnitude on a timescale comparable to the advection timescale of the wake downstream from the bow shock. Good correspondence is found between 2D and 3D simulations with comparable resolution, and 3D simulations can achieve the required resolution with reasonable computing resources. Development of the KH instability is inhibited for shear flows parallel to the ISM magnetic field, compared with what is seen in the perpendicular direction, resulting in synthetic IR emission maps of bow shocks that are smooth when seen from one perspective but show strong distortions from another. Measuring the X-ray morphology and luminosity in bow shocks may be useful for constraining mixing and energy-transfer rates between hot and warm gas phases of the ISM. Dynamical instabilities at the wind-ISM interface are a crucial ingredient in determining the properties of the hot-gas phase in stellar bow-shocks, in particular to capture its time dependence.