Clumped stellar winds in supergiant high-mass X-ray binaries

TL;DR

This paper investigates how clumping in supergiant star winds affects accretion variability and X-ray emission in high-mass X-ray binaries, emphasizing the importance of wind inhomogeneities in modeling and observations.

Contribution

It introduces a time-dependent hydrodynamical model of wind clumping and its impact on accretion and photoionization in supergiant high-mass X-ray binaries.

Findings

Wind clumping causes significant variability in X-ray light curves.

Clumping influences the ionization state of the stellar wind.

Inhomogeneous winds require different modeling approaches for accurate spectra.

Abstract

The clumping of massive star winds is an established paradigm, which is confirmed by multiple lines of evidence and is supported by stellar wind theory. We use the 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 on to 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. Photoionization of inhomogeneous winds is different from the photoinization of smooth winds. The degree of ionization is affected by the wind clumping. The wind clumping must also be taken into account when comparing the observed and model spectra of the photoionized stellar wind.