Stellar wind structures in the eclipsing binary system IGR J18027-2016

TL;DR

This study investigates the stellar wind structures and their effects in the high-mass X-ray binary IGR J18027-2016 by analyzing X-ray spectral and temporal data, revealing variable absorption and wind interaction phenomena.

Contribution

It provides a detailed spectral and temporal analysis of IGR J18027-2016, proposing the roles of photo-ionisation and accretion wakes in shaping observed X-ray properties.

Findings

Absorption column density varies significantly during eclipse phases.

Spectral analysis indicates presence of Fe K lines and variable absorption.

Hardening of X-ray emission observed during eclipse ingress and egress.

Abstract

IGR J18027-2016 is an obscured high-mass X-ray binary formed by a neutron star accreting from the wind of a supergiant companion with a $\sim$4.57 day orbital period. The source shows an asymmetric eclipse profile that remained stable across several years. We aim at investigating the geometrical and physical properties of stellar wind structures formed by the interaction between the compact object and the supergiant star. In this work we analyse the temporal and spectral evolution of this source along its orbit using six archival XMM-Newton observations and the accumulated Swift/BAT hard X-ray light curve. XMM-Newton light curves show that the source hardens during the ingress and egress of the eclipse, in accordance with the asymmetric profile seen in Swift/BAT data. A reduced pulse modulation is observed on the ingress to the eclipse. We model XMM-Newton spectra by means of a thermally-comptonized continuum (nthcomp) adding two gaussian emission lines corresponding to Fe K$\alpha$ and Fe K$\beta$. We included two absorption components to account for the interstellar and intrinsic media. We found that the local absorption column outside the eclipse fluctuates uniformly around $\sim$ 6$\times$10$^{22}$~cm$^{-2}$, whereas, when the source enters and leaves the eclipse, the column increases by a factor of $\gtrsim$3, reaching values up to $\sim$35 and $\sim$15$\times 10^{22}$~cm$^{-2}$, respectively. Combining the physical properties derived from the spectral analysis, we propose a scenario where a photo-ionisation wake (mainly) and an accretion wake (secondarily) are responsible for the orbital evolution of the absorption column, the continuum emission and the variability seen at the Fe-line complex.