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

Abstract

Context. 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 ∼4.57 d orbital period. The source shows an asymmetric eclipse profile that remained stable across several years. Aims. We aim to investigate the geometrical and physical properties of stellar wind structures formed by the interaction between the compact object and the supergiant star. Methods. In this work we analysed 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. Results. The 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 modelled XMM-Newton spectra by means of a thermally Comptonized continuum (NTHCOMP), adding two Gaussian emission lines corresponding to Fe Kα and Fe Kβ. 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 ∼6 × 1022 cm-2, whereas when the source enters and leaves the eclipse the column increases by a factor of 3, reaching values up to ∼35 and ∼15 × 1022 cm-2, respectively. Conclusions. Combining the physical properties derived from the spectral analysis, we propose a scenario in which, primarily, a photo-ionisation wake and, secondarily, an accretion wake are responsible for the orbital evolution of the absorption column, continuum emission, and variability seen at the Fe-line complex.