F. Marin

Modeling optical and UV polarization of AGNs - II. Polarization imaging and complex reprocessing

Context. The innermost parts of active galactic nuclei (AGNs) are believed to be comprised of several emission and scattering media coupled by radiative processes. These regions generally cannot be spatially resolved. Spectropolarimetric observations give important information about the reprocessing geometry. Aims. We aim to obtain a coherent model of the polarization signature resulting from the radiative coupling between the components, to compare our results with polarimetry of thermal AGNs, and thereby to put constraints on the geometry. Methods. We used a new public version of stokes, a Monte Carlo radiative transfer code presented in the first paper of this series. The code has been significantly improved for computational speed and polarization imaging has been implemented. The imaging capability helps to improve understanding of the contributions of different components to the spatially-integrated flux. We coupled continuum sources with a variety of reprocessing regions such as equatorial scattering regions, polar outflows, and toroidal obscuring dust and studied the resulting polarization. We explored combinations and computed a grid of thermal AGN models for different halfopening angles of the torus and polar winds. We also considered a range of optical depths for equatorial and polar electron scattering and investigated how the model geometry influences the type-1/type-2 polarization dichotomy for thermal AGNs (type-1 AGNs tend to be polarized parallel to the axis of the torus while type-2 AGNs tend to be polarized perpendicular to it). Results. We put new constrains on the inflowing medium within the inner walls of the torus. To reproduce the observed polarization in type-1 objects, the inflow should be confined to the common equatorial plane of the torus and the accretion disk and have a radial optical depth of 1 60 ◦ to match the expected level of perpendicular polarization. If outflows are collimated by the torus inner walls, they must not be optically thick ( τ 0.3.

Are there reliable methods to estimate the nuclear orientation of Seyfert galaxies

Orientation, together with accretion and evolution, is one of the three main drivers in the Grand Unification of Active Galactic Nuclei (AGN). Being unresolved, determining the true inclination of those powerful sources is always difficult and indirect, yet it remains a vital clue to apprehend the numerous, panchromatic and complex spectroscopic features we detect. There are only a hundred inclinations derived so far; in this context, can we be sure that we measure the true orientation of AGN? To answer this question, four methods to estimate the nuclear inclination of AGN are investigated and compared to inclination-dependent observables (hydrogen column density, Balmer linewidth, optical polarization, and flux ratios within the IR and relative to X-rays). Among these orientation indicators, the method developed by Fisher, Crenshaw, Kraemer et al., mapping and modeling the radial velocities of the [O iii] emission region in AGN, is the most successful. The [O iii]-mapping technique shows highly statistically significant correlations at >95% confidence level for rejecting null hypothesis for all the test cases. Such results confirm that the Unified Model is correct at a scale ranging from kiloparsec to a fraction of a parsec. However, at a radial distance less than 0.01 pc from the central black hole, warps and misalignments may change this picture.

Modeling optical and UV polarization of AGNs - III. From uniform-density to clumpy regions

A growing body of evidence suggests that part of, if not all, scattering regions of active galactic nuclei (AGNs) are clumpy. Hence. in this paper, we run radiative transfer models in the optical/UV for a variety of AGN reprocessing regions with different distributions of clumpy scattering media. We use the latest version of the Monte Carlo code STOKES presented in the first two papers of this series to model AGN reprocessing regions of increasing morphological complexity. We replace previously uniform-density media with up to thousands of constant-density clumps. We couple a continuum source to fragmented equatorial scattering regions, polar outflows, and toroidal, obscuring dust regions and investigate a wide range of geometries. We also consider different levels of fragmentation in each scattering region to evaluate importance of fragmentation for the net polarization of the AGN. We find that, in comparison with uniform-density models, equatorial distributions of gas and dust clouds result in grayer spectra, and show a decrease of the net polarization percentage at all lines of sight. The resulting polarization position angle depends on the morphology of the clumpy structure, with extended tori favoring parallel polarization while compact tori produce orthogonal polarization position angles. In the case of polar scattering regions, fragmentation increases the net polarization unless the cloud filling factor is small. A complete set of AGN models constructed from the individual, fragmented regions is investigated. Our modeling shows that the introduction of fragmented dusty tori significantly alters the resulting net polarization of an AGN. Comparison of our models to polarization observations of large AGN samples greatly favors geometrically compact clumpy tori over extended ones.

X-ray polarimetry as a new tool to discriminate reflection from absorption scenarios — predictions for MCG-6-30-15

We present modelling of X-ray polarization spectra emerging from the two competing scenarios that are proposed to explain the broad Fe Kα line in the Seyfert 1 galaxy MCG-6-30-15. The polarization signature of complex absorption is studied for a partial covering scenario using a clumpy wind and compared to a reflection model based on the lamppost geometry. The shape of the polarization percentage and angle as a function of photon energy are found to be distinctly different between the reflection and the absorption cases. Relativistic reflection produces significantly stronger polarization in the 1–10 keV energy band than absorption. The spectrum of the polarization angle adds additional constraints: in the absorption case it shows a constant shape, whereas the relativistic reflection scenario typically leads to a smooth rotation of the polarization angle with photon energy. Based on this work, we conclude that a soft X-ray polarimeter onboard a small X-ray satellite may already discriminate between the absorption and the reflection scenarios. A promising opportunity may arise with the X-ray Imaging Polarimetry Explorer mission, which has been proposed to the European Space Agency in response to a small-size (S-class) mission call due for launch in 2017.

Reflection nebulae in the Galactic center: soft X-ray imaging polarimetry

The origin of irradiation and fluorescence of the 6.4 keV bright giant molecular clouds surrounding Sgr A*, the central supermassive black hole of our Galaxy, remains enigmatic. Testing the theory of a past active period of Sgr A* requires X-ray polarimetry. In this paper, we show how modern imaging polarimeters could revolutionize our understanding of the Galactic Center. Through Monte Carlo modeling, we produce a 4-8 keV polarization map of the Galactic Center, focusing on the polarimetric signature produced by Sgr B1, Sgr B2, G0.11-0.11, Bridge E, Bridge D, Bridge B2, MC2, MC1, Sgr C3, Sgr C2, and Sgr C1. We estimate the resulting polarization, include polarized flux dilution by the diffuse plasma emission detected toward the GC, and simulate the polarization map that modern polarimetric detectors would obtain assuming the performances of a mission prototype. The eleven reflection nebulae investigated in this paper present a variety of polarization signatures, ranging from nearly unpolarized to highly polarized (about 77%) fluxes. A major improvement in our simulation is the addition of a diffuse, unpolarized plasma emission that strongly impacts soft X-ray polarized fluxes. The dilution factor is in the range 50% - 70%, making the observation of the Bridge structure unlikely even in the context of modern polarimetry. The best targets are the Sgr B and Sgr C complexes, and the G0.11-0.11 cloud. An exploratory observation of a few hundred kilo-seconds of the Sgr B complex would allow a significant detection of the polarization and be sufficient to derive hints on the primary source of radiation. A more ambitious program (few Ms) of mapping the giant molecular clouds could then be carried out to probe with great precision the turbulent history of Sgr A*, and place important constraints on the composition and three-dimensional position of the surrounding gas.

A structure for quasars under the scope of polarization – I. The UV/optical polarization dichotomy of type-1 and type-2 AGN

We present UV/optical spectropolarimetric modelling of the phenomenologically-based structure for quasars proposed by Elvis (2000). In this first paper of a series, we explore the continuum polarisation emerging from radiatively accelerated and bent winds that were vertically launched from the accretion disc in an active galactic nucleus (AGN). We simulate the radiative transfer occurring in Thomson scattering and dust extinction media over a range of morphological parameters and optical depths of the wind. We demonstrate that the wind geometry proposed by Elvis with a phenomenologically-derived bending angle of theta = 60deg still underestimates the observed optical polarisation percentage of type-1 and type-2 AGN and does not yet reproduce the expected dichotomy of the polarisation position angle. To recover the observed polarisation properties, a smaller bending angle and some amount of dust shielding in the equatorial region should be considered. A two-phase outflow is found to generate both the observed polarisation dichotomy and acceptable levels of polarisation degree if the wind has a bending angle theta = 45deg, and the conical shells have a half-openingangle of 3deg < delta_theta < 10deg. The absorbing dust column at the wind base should be in the range of 1 < tau_dust < 4 (tau being integrated over 2000 - 8000 Angs). Straightforward observational tests from spectropolarimetry and from determining the number density of different AGN types can be performed to further constrain the wind geometry.

Off-axis irradiation and the polarization of broad emission lines in active galactic nuclei

Abstract The stokes Monte Carlo radiative transfer code has been extended to model the velocity dependence of the polarization of emission lines. We use stokes to present improved modeling of the velocity-dependent polarization of broad emission lines in active galactic nuclei. We confirm that off-axis continuum emission can produce observed velocity dependencies of both the degree and position angle of polarization. The characteristic features are a dip in the percentage polarization and an S-shaped swing in the position angle of the polarization across the line profile. Some differences between our stokes results and previous modeling of polarization due to off-axis emission are noted. In particular we find that the presence of an offset between the maximum in line flux and the dip in the percentage of polarization or the central velocity of the swing in position angle does not necessarily imply that the scattering material is moving radially. Our model is an alternative scenario to the equatorial scattering disk described by Smith et al. (2005). We discuss strategies to discriminate between both interpretations and to constrain their relative contributions to the observed velocity-resolved line and polarization.

Predicting the X-ray polarization of type 2 Seyfert galaxies

Infrared, optical and ultraviolet spectropolarimetric observations have proven to be ideal tools for the study of the hidden nuclei of type-2 active galactic nuclei (AGN) and for constraining the composition and morphology of the sub-parsec scale emission components. In this paper, we extend the analysis to the polarization of the X-rays from type-2 AGN. Combining two radiative transfer codes, we performed the first simulations of photons originating in the gravity dominated vicinity of the black hole and scattering in structures all the way out to the parsec-scale torus and polar winds. We demonstrate that, when strong gravity effects are accounted for, the X-ray polarimetric signal of Seyfert-2s carries as much information about the central AGN components as spectropolarimetric observations of Seyfert-1s. The spectropolarimetric measurements can constrain the spin of the central supermassive black hole even in edge-on AGN, the hydrogen column density along the observers line-of-sight, and the composition of the polar outflows. However, the polarization state of the continuum source is washed out by multiple scattering, and should not be measurable unless the initial polarization is exceptionally strong. Finally, we estimate that modern X-ray polarimeters, either based on the photo-electric effect or on Compton scattering, will require long observational times on the order of a couple of mega-seconds to be able to properly measure the polarization of type-2 AGN.

X-ray polarimetric signatures induced by spectral variability in the framework of the receding torus model

Obscuring circumnuclear dust is a well-established constituent of active galactic nuclei (AGN). Traditionally referred to as the receding dusty torus, its inner radius and angular extension should depend on the photo-ionizing luminosity of the central source. Using a Monte Carlo approach, we simulate the radiative transfer between the multiple components of an AGN adopting model constraints from the bright Seyfert galaxy NGC 4151. We compare our model results to the observed near-IR to UV polarization of the source and predict its X-ray polarization. We find that the 2-8 keV polarization fraction of a standard AGN model varies from less then a few percent along polar viewing angles up to tens of percent at equatorial inclinations. At viewing angles around the type-1/type-2 transition the X-ray polarization variability differs between a static or a receding torus scenario. In the former case, the expected 2-8 keV polarization of NGC 4151 is found to be 1.21% +/- 0.34% with a constant polarization position angle, while in the later scenario it varies from 0.1% to 6% depending on the photon index of the primary radiation. Additionally, an orthogonal rotation of the polarization position angle with photon energy appears for very soft primary spectra. Future X-ray polarimetry missions will be able to test if the receding model is valid for Seyfert galaxies seen at a viewing angle close to the torus horizon. The overall stability of the polarization position angle for photon indexes softer than {\Gamma} = 1.5 ensures that reliable measurements of X-ray polarization are possible. We derive a long-term observational strategy for NGC 4151 assuming observations with a small to medium-size X-ray polarimetry satellite.

Prospects of 3D mapping of the Galactic Centre clouds with X-ray polarimetry

Despite past panchromatic observations of the innermost part of the Milky Way, the overall structure of the Galactic Centre (GC) remains enigmatic in terms of geometry. In this paper, we aim to show how polarimetry can probe the three-dimensional position of the molecular material in the central ~100 pc of the GC. We investigate a model where the central supermassive black hole Sgr A* is radiatively coupled to a fragmented circumnuclear disc (CND), an elliptical twisted ring representative of the central molecular zone (CMZ), and the two main, bright molecular clouds Sgr B2 and Sgr C. 8 -- 35 keV integrated polarization mapping reveals that Sgr B2 and Sgr C, situated at the two sides of the CMZ, present the highest polarization degrees (66.5 and 47.8 per cent respectively), both associated with a polarization position angle psi = 90deg (normal to the scattering plane). The CND shows a lower polarization degree, 1.0 per cent with psi = -20.5deg, tracing the inclination of the CND with respect to the Galactic plane. The CMZ polarization is spatially variable. We also consider a range of spatial locations for Sgr A* and the reprocessing media, and investigate how the modeled three-dimensional geometry influences the resulting GC polarization. The two reflection nebulae are found to always produce high polarization degrees (>> 10 per cent). We show that a 500 ks observation with a broadband polarimeter could constrain the location and the morphology of the scattering material with respect to the emitting source, revealing the past activity of Sgr A*.