Color-color diagrams as tools for assessment of the variable absorption in high mass X-ray binaries

TL;DR

This paper demonstrates how color-color diagrams can effectively assess variable absorption in high mass X-ray binaries, providing insights into stellar wind structures despite short timescale limitations.

Contribution

It introduces a method using color-color diagrams with partial covering models to evaluate variable absorption, emphasizing the importance of ionization effects.

Findings

Color-color diagrams can track absorption variability in high mass X-ray binaries.

Ionization state variations significantly influence the shape of color-color tracks.

Accounting for ionization improves the modeling of observed spectral behavior.

Abstract

High mass X-ray binaries hold the promise of giving us understanding of the structure of the winds of their supermassive companion stars by using the emission from the compact object as a backlight to evaluate the variable absorption in the structured stellar wind. The wind along the line of sight can change on timescales as short as minutes and below. However, such short timescales are not available to direct measurement of absorption through X-ray spectroscopy with the current generation of X-ray telescopes. In this paper, we demonstrate the usability of color-color diagrams for assessing the variable absorption in wind accreting high mass X-ray binary systems. We employ partial covering models to describe the spectral shape of high mass X-ray binaries and assess the implication of different absorbers and their variability on the shape of color-color tracks. We show that taking into account the ionization of the absorber, and in particular accounting for the variation of ionization with absorption depth, is crucial to describe the observed behavior well.