# Variability in high-mass X-ray binaries

## TL;DR

This paper explores the variability in high-mass X-ray binaries caused by neutron star rotation and stellar wind clumps, providing insights into accretion physics and wind properties through spectral and timing analysis.

## Contribution

It offers a detailed analysis of variability mechanisms in high-mass X-ray binaries, emphasizing the importance of viewing-angle-resolved models and wind structure characterization.

## Key findings

- Neutron star rotation causes significant changes in observed accretion regions.
- Clumpy stellar winds induce variability in luminosity and absorption.
- Variability analysis reveals properties of accretion columns and stellar winds.

## Abstract

Strongly magnetized, accreting neutron stars show periodic and aperiodic variability over a wide range of time scales. By obtaining spectral and timing information on these different time scales, we can have a closer look into the physics of accretion close to the neutron star and the properties of the accreted material. One of the most prominent time scales is the strong pulsation, i.e., the rotation period of the neutron star itself. Over one rotation, our view of the accretion column and the X-ray producing region changes significantly. This allows us to sample different physical conditions within the column but at the same time requires that we have viewing-angle-resolved models to properly describe them. In wind-fed high-mass X-ray binaries, the main source of aperiodic variability is the clumpy stellar wind, which leads to changes in the accretion rate (i.e., luminosity) as well as absorption column. This variability allows us to study the behavior of the accretion column as a function of luminosity, as well as to investigate the structure and physical properties of the wind, which we can compare to winds in isolated stars.