Issue |
A&A
Volume 689, September 2024
|
|
---|---|---|
Article Number | A297 | |
Number of page(s) | 16 | |
Section | Numerical methods and codes | |
DOI | https://doi.org/10.1051/0004-6361/202450773 | |
Published online | 19 September 2024 |
X-ray polarisation in AGN circumnuclear media
Polarisation framework and 2D torus models
1
Sterrenkundig Observatorium, Universiteit Gent,
Krijgslaan 281 S9,
9000
Gent,
Belgium
2
Institut d’Astrophysique et de Géophysique, Université de Liège,
Allée du 6 Août 19c,
4000
Liège,
Belgium
3
Astronomical Observatory,
Volgina 7,
11060
Belgrade,
Serbia
4
Dipartimento di Matematica e Fisica, Università degli Studi Roma Tre,
via della Vasca Navale 84,
00146
Roma,
Italy
Received:
17
May
2024
Accepted:
29
July
2024
Context. Cold gas and dust reprocess the central X-ray emission of active galactic nuclei (AGN), producing characteristic spectro-polarimetric features in the X-ray band. The recent launch of IXPE allows for observations of this X-ray polarisation signal, which encodes unique information on the parsec-scale circumnuclear medium of obscured AGN. However, the models for interpreting these polarimetric data are under-explored and do not reach the same level of sophistication as the corresponding spectral models.
Aims. We aim at closing the gap between the spectral and spectro-polarimetric modelling of AGN circumnuclear media in the X-ray band by providing the tools for simulating X-ray polarisation in complex geometries of cold gas alongside X-ray spectra.
Methods. We lay out the framework for X-ray polarisation in 3D radiative transfer simulations and provide an implementation to the 3D radiative transfer code SKIRT, focussing on (de)polarisation due to scattering and fluorescent re-emission. As an application, we explored the spectro-polarimetric properties of a 2D toroidal reprocessor of cold gas, modelling the circumnuclear medium of AGN.
Results. For the 2D torus model, we find a complex behaviour of the polarisation angle with photon energy, which we interpret as a balance between the reprocessed photon flux originating from different sky regions, with a direct link to the torus geometry. We calculated a large grid of AGN torus models and demonstrated how spatially resolved X-ray polarisation maps could form a useful tool for interpreting the geometrical information that is encoded in IXPE observations. With this work, we release high-resolution AGN torus templates that simultaneously describe X-ray spectra and spectro-polarimetry for observational data fitting with XSPEC.
Conclusions. The SKIRT code can now model X-ray polarisation simultaneously with X-ray spectra and provide synthetic spectro-polarimetric observations for complex 3D circumnuclear media, with all features of the established SKIRT framework available.
Key words: polarization / radiative transfer / methods: numerical / galaxies: nuclei / X-rays: general
© The Authors 2024
Open Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
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