Clumped stellar winds in supergiant high-mass X-ray binaries: X-ray variability and photoionization

TL;DR

This paper models the effects of wind clumping in supergiant high-mass X-ray binaries, revealing how inhomogeneities influence X-ray variability, absorption, and ionization, and highlighting the importance of clumping in interpreting observations.

Contribution

It bridges detailed wind models with phenomenological descriptions in HMXBs, quantifies the impact of clumping on X-ray variability and photoionization, and introduces correction factors for models.

Findings

Wind clumping causes significant X-ray variability up to eight orders of magnitude.

Absorption of X-rays varies strongly with orbital phase due to wind inhomogeneities.

Clumping reduces the photoionization parameter, affecting spectral modeling.

Abstract

The clumping of massive star winds is an established paradigm confirmed by multiple lines of evidence and supported by stellar wind theory. The purpose of this paper is to bridge the gap between detailed models of inhomogeneous stellar winds in single stars and the phenomenological description of donor winds in supergiant high-mass X-ray binaries (HMXBs). We use results from time-dependent hydrodynamical models of the instability in the line-driven wind of a massive supergiant star to derive the time-dependent accretion rate onto a compact object in the Bondi-Hoyle-Lyttleton approximation. The strong density and velocity fluctuations in the wind result in strong variability of the synthetic X-ray light curves. The model predicts a large scale X-ray variability, up to eight orders of magnitude, on relatively short timescales. The apparent lack of evidence for such strong variability in the observed HMXBs indicates that the details of accretion process act to reduce the variability due to the stellar wind velocity and density jumps. We, also, study the absorption of X-rays in the clumped stellar wind by means of a 2-D stochastic wind model and find that absorption of X-rays changes strongly at different orbital phases. Furthermore, we address the photoionization in the clumped wind, and show that the degree of ionization is affected by the wind clumping. A correction factor for the photoionization parameter is derived. It is shown that the photoionization parameter is reduced by a factor Xi compared to the smooth wind models with the same mass-loss rate, where Xi is the wind inhomogeneity parameter. We conclude that wind clumping must also be taken into account when comparing the observed and model spectra of the photoionized stellar wind.