Modelling interstellar structures around Vela X-1

TL;DR

This study discovers filamentary structures behind Vela X-1's bow shock, uses optical spectroscopy and 3D magnetohydrodynamic simulations to analyze the interaction with interstellar medium, highlighting the importance of stellar wind magnetic fields.

Contribution

It introduces detailed 3D MHD simulations with different magnetic field configurations to explain the observed structures around Vela X-1, emphasizing the role of stellar wind magnetization.

Findings

Magnetized stellar wind with a helical field best reproduces observed structures.

Hydrodynamic and simple magnetic models only approximate the observations.

Magnetic fields significantly influence bow shock and wake morphology.

Abstract

We report the discovery of filamentary structures stretched behind the bow-shock-producing high-mass X-ray binary Vela X-1 using the SuperCOSMOS H-alpha Survey and present the results of optical spectroscopy of the bow shock carried out with the Southern African Large Telescope (SALT). The geometry of the detected structures suggests that Vela X-1 has encountered a wedge-like layer of enhanced density on its way and that the shocked material of the layer partially outlines a wake downstream of Vela X-1. To substantiate this suggestion, we carried out 3D magnetohydrodynamic simulations of interaction between Vela X-1 and the layer for three limiting cases. Namely, we run simulations in which (i) the stellar wind and the interstellar medium (ISM) were treated as pure hydrodynamic flows, (ii) a homogeneous magnetic field was added to the ISM, while the stellar wind was assumed to be unmagnetized, and (iii) the stellar wind was assumed to possess a helical magnetic field, while there was no magnetic field in the ISM. We found that although the first two simulations can provide a rough agreement with the observations, only the third one allowed us to reproduce not only the wake behind Vela X-1, but also the general geometry of the bow shock ahead of it.