Footprints in the wind of Vela X-1 traced with MAXI

TL;DR

This paper investigates how X-ray photo-ionization affects the stellar wind in Vela X-1, revealing a complex wind structure with a denser wake, using MAXI spectral data to challenge the assumption of a smooth, spherical wind.

Contribution

The study introduces a simple model of a wind with a stream-like photoionization wake, improving understanding of wind structure in high mass X-ray binaries.

Findings

Observed absorption inconsistent with spherical wind models

Identified a denser, slower wind region trailing the neutron star

Proposed a wake structure to explain absorption patterns

Abstract

The stellar wind around the compact object in luminous wind-accreting high mass X-ray binaries is expected to be strongly ionized with the X-rays coming from the compact object. The stellar wind of hot stars is mostly driven by light absorption in lines of heavier elements, and X-ray photo-ionization significantly reduces the radiative force within the so-called Stroemgren region leading to wind stagnation around the compact object. In close binaries like Vela X-1 this effect might alter the wind structure throughout the system. Using the spectral data from Monitor of All-sky X-ray Image (MAXI), we study the observed dependence of the photoelectric absorption as function of orbital phase in Vela X-1, and find that it is inconsistent with expectations for a spherically-symmetric smooth wind. Taking into account previous investigations we develop a simple model for wind structure with a stream-like photoionization wake region of slower and denser wind trailing the neutron star responsible for the observed absorption curve.