Spotting the misaligned outflows in NGC 1068 using X-ray polarimetry

TL;DR

This study models X-ray polarisation in NGC 1068 to determine the orientation of ionised outflows and their misalignment with the torus axis, proposing future X-ray polarimetry as a diagnostic tool.

Contribution

It introduces a comprehensive radiative transfer model to predict X-ray polarisation signatures that reveal outflow misalignments in active galactic nuclei.

Findings

Polarisation angle rotation indicates outflow misalignment.

X-ray polarimetry can verify the outflow-torus orientation.

Misalignment can be constrained with future broad-band polarimeters.

Abstract

We model the expected X-ray polarisation induced by complex reprocessing in the active nucleus of the Seyfert-2 galaxy NGC 1068. Recent analysis of infrared interferometry observations suggests that the ionised outflows ejected by the central engine are not aligned with the symmetry axis of the obscuring torus. This conclusion was obtained by extrapolating the apparent orientation of the narrow line region to the inner parts of the ionisation cones. We show that future measurements of the soft X-ray polarisation vector unambiguously determine the orientation of the ionisation cones. Furthermore, X-ray polarimetry across a broad photon energy range may independently verify the misalignment between the ionisation cones and the axis of the torus. To model the expected polarisation percentage and position angle, we apply the radiative transfer code STOKES. Reprocessing of the primary X-ray radiation takes place in the accretion disc, the surrounding equatorial torus and the inclined, ionised outflows. We also examine additional equatorial scattering occurring in between the accretion disc and the inner surfaces of the torus. Radiative coupling between the different reprocessing components is computed coherently. The resulting polarisation properties depend on the optical depth of the reprocessing regions and on the viewing angle of the observer. We show that even under unfavourable conditions the misalignment of the outflows with respect to the torus axis can be determined from a rotation of the polarisation position angle between softer and harder X-rays. We argue that the misalignment of the outflows with respect to the torus axis in NGC 1068 may be constrained by a future X-ray mission if equipped with a broad band polarimeter.