Multi-wavelength radiative transfer and polarization modeling of active galactic nuclei and related objects – POLIOPTIX

We follow a combined theoretical and observational approach. The optical/UV spectral range is observationally explored by spectropolarimetric observations; the study of the X-ray range is based on on spectroscopic results obtained, for instance, by the XMM-Newton satellite. Our analysis also includes the infrared domain using public archives of spectroscopic data. An important motivation of this project is to establish perspectives for X-ray polarimetry. Our work aims to deliver reliable simulations that allow us to constrain the performance of future space-borne X-ray polarimeters. To theoretically explore the complex emission and scattering geometry in the vicinity of a supermassive black hole we apply the code STOKES that is designed to model the polarization of active galactic nuclei. To take into account General Relativity effects on the radiation passing close to the black hole, the STOKES results are combined with a relativistic ray-tracing method. This modeling work allows us to obtain a much more detailed image of the geometry and the dynamics close to an accreting supermassive black hole.

We completed the calculation of a first polarization model grid for AGN in the optical and UV spectral range. Based on the observations of a number of AGN published in the literature, this model grid allowed us to put important constraints on the opacity of the presumed accretion and ejection flow in Seyfert galaxies.

In the following, it was shown that our models describe the observed optical/UV polarization in a satisfactory way. In particular, it was possible to correctly reproduce the behavior of the observed polarization position angle as a function of the object's classification as a type-1 or type-2.

Aside from modeling the continuum emission, we started to study the spectral and polarization profile of the broad optical AGN lines. Applying an asymmetric irradiation geometry we manage to reproduce the observed spectropolarimetric data in a rather straightforward manner. The implications of our model are important for our understanding of accretion disks of which we thus start to explore sub-orbital structures (figure 1).

In the X-ray spectral range, our results contribute to an important debate about the relevance of relativistic effects in the formation of broad iron Ka lines observed in a number of AGN. We showed that an X-ray polarimetry observation could determine which of the two proposed scenarios is correct: if the line broadening is due to relativistic effects, the polarization signal has a significant degree and its polarization position angle varies as a function of the photon energy across the line. If, on the other hand, the broad line is due to partial covering in an absorbing, polar wind the X-ray polarization remains weak and has a constant position angle (figure 2).

We currently start to extend our polarization modeling of AGN into the infrared, optical and UV bands while including more complex and non-symmetric scenarios for the geometry. In the X-ray domain, the detailed polarization modeling emerging from irradiated accretion disks continues. In the same context, we start to model the wind geometry suggested by the hydrodynamical simulations of accretion and ejection flows. Furthermore, we are soon going to start the comparison of models to the observations. These observations include new spectropolarimetry data taken at ESO's Very Large Telescope and data from the archives of ESA's XMM-Newton space observatory.

Publications in international refereed journals:

- Marin, F., Goosmann, R. W., Gaskell, C. M., Porquet, D., Dovciak, M., 2012, Astronomy & Astrophysics, 548A, 121

- Gaskell C. M., Goosmann, R. W., Astrophysical Journal 769, 30

- Marin, F., Goosmann, R. W., Dovciak, M., Muleri, F., Porquet, D., Grosso, N., Karas, V, Matt, G., 2012, Monthly Notices of the Royal Astronomical Society Letters, Volume 426, Issue 1, pp. L101-L105

- Marin, F., Porquet, D., Goosmann, R. W., Dovciak, M., Muleri, F., Grosso, N., Karas, V., Monthly Notices of the Royal Astronomical Society, Volume 436, Issue 2, p.1615-1620

- Marin, F. & Goosmann, R. W., Monthly Notices of the Royal Astronomical Society, Volume 436, Issue 3, p.2522-2534

- Marin, F., Monthly Notices of the Royal Astronomical Society, Volume 441, Issue 1, p.551-564

Publication in international, refereed conference proceedings:

- Marin, F., Tamborra, F., Volume 54, Issue 7, p. 1458-1466

- Goosmann, R. W., Gaskell, C. M., Marin, F., 2013, Advances in Space Research, Volume 54, Issue 7, p. 1341-1346

Publication in national, non-refereed conference proceedings:

- Marin, F.; Goosmann, R. W., 2012, Proceedings of the Annual meeting of the French Society of Astronomy and Astrophysics, pp.587-590

- Marin, F.; Goosmann, R. W., 2013, Proceedings of the Annual meeting of the French Society of Astronomy and Astrophysics, pp.475-478

- Marin, F.; Goosmann, R. W., 2013, Proceedings of the Annual meeting of the French Society of Astronomy and Astrophysics, pp.479-482

- Marin, F.; Goosmann, R. W., 2014, Proceedings of the Annual meeting of the French Society of Astronomy and Astrophysics, in press

We also put on-line a new, publicly available version 1.2 of the STOKES code.

Even the closest active galactic nuclei (AGN) cannot be spatially resolved with the current observational technology. Our understanding of the complicated structure of AGN is therefore gathered from indirect conclusions on, mostly, spectroscopy results. The geometry and dynamics in the vicinity of accreting supermassive black holes must be complicated but the details of the accretion and ejection flows, of the obscuring equatorial dust or the line emission regions are still poorly constrained. Various emission and scattering regions are coupled by radiative processes, and at the very smallest distances to the last stable orbit of the black hole, relativistic effects on the (X-ray) radiation come into play.
In this proposal we suggest to examine the complexity of the inner region of radio-quiet AGN by exploring the observational information over a broad wavelength range and by including as many observables as possible. The goal of the four-year project we propose is to investigate multi-wavelength polarization, spectroscopy and timing properties of AGN. The wavelength range to cover includes the infrared, optical, ultraviolet and X-ray range simultaneously. The model will include the effects of fragmentation (clumpiness) in the scattering medium of the torus and the broad line region. This is going to be the most advanced and complete radiative transfer model for AGN on the market.
Our strategy contains both, extensive theoretical modeling and detailed analysis of observational data. For the optical/UV band we consider spectropolarimetry observations, for the X-ray range spectroscopy results obtained with XMM-Newton. Archival spectroscopy results will also be considered in the infrared band. Nevertheless – the certain perspective of upcoming modern X-ray polarimetry missions, with planned launch days as early as 2015, is a key motivation for our proposal. Our work aims to provide reliable simulations that for now, pre-launch of the first X-ray polarimetry satellite, constrain the expected scientific performance of AGN observations. Post-launch, our modeling can serve to interpret the first real X-ray polarization measurements of AGN.
Our consortium includes experienced researchers in radiative transfer modeling and X-ray data analysis. Furthermore, we have established stable and reliable international collaborations with experts in general relativity, in optical/UV spetropolarimetry of AGN, and in the development of X-ray polarimetry instrumentation. The project takes profit and is designed to create synergies between these various domains of expertise. It supports the emerging research field of AGN in the High Energy Group of Strasbourg Observatory.
The objectives of this project are built on previous observational and modeling results. For the theoretical exploration of complex emission and scattering geometries close to supermassive black holes we apply the radiative transfer code STOKES that has already been used to model AGN polarization. The code can handle time-dependent effects. To include relativistic effects on the X-ray radiation very close to the black hole, the results of STOKES are coupled to the relativistic ray-tracing code KY. We expect to obtain a much more detailed picture of the geometry and dynamics close to accreting supermissive black holes. In addition to that some results will be transferable to related object classes such as Galactic black hole candidates in X-ray binaries.