Journal Article

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

R. W. Goosmann and G. Matt

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 415, issue 4, pages 3119-3128
Published in print August 2011 | ISSN: 0035-8711
Published online August 2011 | e-ISSN: 1365-2966 | DOI:
Spotting the misaligned outflows in NGC 1068 using X-ray polarimetry

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We model the expected X-ray polarization induced by complex reprocessing in the active nucleus of the Seyfert 2 galaxy NGC 1068. Recent analysis of infrared interferometry observations suggests that the ionized 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 ionization cones. We show that future measurements of the soft X-ray polarization vector unambiguously determine the orientation of the ionization cones. Furthermore, X-ray polarimetry across a broad photon energy range may independently verify the misalignment between the ionization cones and the axis of the torus. To model the expected polarization 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, ionized 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 polarization 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 polarization 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.

Keywords: polarization; radiative transfer; techniques: polarimetric; galaxies: individual: NGC 1068; galaxies: Seyfert; X-rays: galaxies

Journal Article.  7523 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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