Thermal emission from bow shocks

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

¹ Astronomy & Astrophysics Section, School of Cosmic Physics, Dublin Institute for Advanced Studies, DIAS Dunsink Observatory, Dublin D15 XR2R, Ireland
² I. Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany
³ Astronomy Unit, School of Physics and Astronomy, Queen Mary University of London, London E1 4NS, UK
⁴ Institute of Physics and Astronomy, University of Potsdam, 14476 Potsdam-Golm, Germany
⁵ School of Physical Sciences and Centre for Astrophysics & Relativity, Dublin City University, Glasnevin D09 W6Y4, Ireland
⁶ ARC Centre of Excellence for the Weather of the 21st Century, University of New South Wales, Sydney, Australia

^★ Corresponding author; jmackey@cp.dias.ie

Received: 10 January 2025
Accepted: 9 March 2025

Abstract

Context. 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 interstellar medium (ISM) by quantifying mixing at the interface between hot and warm gas phases.

Aims. 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.

Methods. A suite of 2D and 3D hydrodynamic and magnetohydrodynamic simulations of bow shocks were run and analysed to generate synthetic emission maps and light curves in X-ray and infrared emission.

Results. 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.

Conclusions. Measuring the X-ray morphology and luminosity in bow shocks may be useful for constraining mixing and energytransfer 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.