Modeling broadband X-ray absorption of massive star winds

TL;DR

This paper introduces a realistic model for calculating X-ray transmission through massive star winds, improving upon previous approximations, and demonstrates its application to observational data revealing wind absorption effects.

Contribution

The authors develop an exact integration method for X-ray transmission in stellar winds, implemented in XSPEC, with a more accurate wind opacity model for analyzing stellar X-ray spectra.

Findings

Wind absorption explains X-ray hardness trends in OB stars.

The model provides a better fit to Chandra data than previous approximations.

Implementation in XSPEC facilitates broader application.

Abstract

We present a method for computing the net transmission of X-rays emitted by shock-heated plasma distributed throughout a partially optically thick stellar wind from a massive star. We find the transmission by an exact integration of the formal solution, assuming that the emitting plasma and absorbing plasma are mixed at a constant mass ratio above some minimum radius, below which there is assumed to be no emission. This model is more realistic than either the slab absorption associated with a corona at the base of the wind or the exospheric approximation that assumes that all observed X-rays are emitted without attenuation from above the radius of optical depth unity. Our model is implemented in XSPEC as a pre-calculated table that can be coupled to a user-defined table of the wavelength dependent wind opacity. We provide a default wind opacity model that is more representative of real wind opacities than the commonly used neutral interstellar medium (ISM) tabulation. Preliminary modeling of \textit{Chandra} grating data indicates that the X-ray hardness trend of OB stars with spectral subtype can largely be understood as a wind absorption effect.