[论文解读] Modeling optical and UV polarization of AGNs II. Polarization imaging and complex reprocessing

Frédéric Marin, R. W. Goosmann+3 more|Sep 13, 2012

Astrophysics and Star Formation Studies参考文献 69被引用 42

一句话总结

本文在STOKES蒙特卡洛辐射转移代码中引入了一种新的偏振成像能力，用于模拟热型活动星系核在光学和紫外波段的偏振特性，整合了来自赤道散射、尘埃环和极向喷流的复杂再处理过程。研究发现，宽角环形结构的半开角（~60°）以及赤道散射光学深度τ ≈ 1–3是重现观测到的偏振二分性的关键因素，其中类型1 AGN表现出平行偏振，而类型2 AGN则呈现垂直偏振，这主要由几何构型和光学深度约束决定。

ABSTRACT

We model the spectropolarimetric signature resulting from the radiative coupling between the innermost parts of active galactic nuclei (AGNs). We use 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. We couple continuum sources with equatorial scattering regions, polar outflows, and toroidal obscuring dust and we study the resulting polarization. We compute a grid of thermal AGN models for different half-opening angles of the torus and polar winds. We also consider a range of optical depths for equatorial and polar electron scattering and investigate how the model geometry influences the type-1/type-2 polarization dichotomy for thermal AGNs. We put new constrains on the inflowing medium within the inner walls of the torus. The inflow should be confined to the common equatorial plane of the torus and the accretion disc and have a radial optical depth of 1 < tau < 3. Our modeling of type-1 AGNs also indicates that the torus is more likely to have a large (~ 60°) half-opening angle. Polarization perpendicular to the axis of the torus may arise at a type-1 viewing angle for a torus half-opening angle of 30°- 45° or polar outflows with an optical depth near unity. Our modeling suggests that most Seyfert-2 AGN must have a half-opening angle > 60° to match the level of perpendicular polarization expected. If outflows are collimated by the torus inner walls, they must not be optically thick (tau < 1) in order to preserve the polarization dichotomy. The wind's optical depth is found not to play a critical role for the degree of polarization of type-2 thermal AGNs but it has a significant impact on the type-1/type-2 polarization dichotomy when the optical depth exceeds tau = 0.3.

研究动机与目标

• 使用物理上一致的辐射转移框架，包含多个再处理组分，对热型AGN的偏振特征进行建模。
• 通过分析几何构型与光学深度对偏振位置角和偏振度的影响，解析类型1/类型2偏振二分性。
• 利用光谱偏振观测约束AGN内区结构，特别是吸积盘、环形结构和喷流的形态。

提出的方法

• 在STOKES蒙特卡洛辐射转移代码中实现偏振成像，支持空间分辨的流量与偏振图生成。
• 对三种关键再处理组分进行建模：赤道电子散射、尘埃环和双锥形极向喷流。
• 基于网格计算方式，对不同半开角（30°–60°）、光学深度（τ < 3）和视线方向进行偏振模拟。
• 采用改进的随机数生成方法与计算优化技术，实现更快收敛与更高保真度的模拟结果。
• 将模型生成的偏振谱与图像与实际AGN光谱偏振观测进行对比，以约束几何构型与光学深度参数。

实验结果

研究问题

• RQ1 为重现类型1 AGN中观测到的平行偏振，所需的环形结构几何形态与光学深度为何？
• RQ2 赤道与极向散射组分如何共同贡献于热型AGN的整体偏振特征？
• RQ3 在环形结构为致密或光滑分布的情况下，能否重现类型2 AGN中观测到的垂直偏振？风的光学深度在此过程中起何作用？
• RQ4 当环形结构与喷流轴线发生非对齐时，对NGC 1068等AGN的偏振分布模式有何影响？
• RQ5 偏振谱对环形结构漏斗区是否存在赤道散射区域提供了何种约束？

主要发现

• 为重现类型1 AGN中观测到的平行偏振，赤道散射区的径向光学深度需满足1 < τ < 3，表明在吸积盘外缘处为光学厚散射。
• 环形结构的半开角约为60°时，才能产生类型1 AGN中观测到的平行偏振；而较窄的开角则更倾向于导致垂直偏振。
• 为匹配类型2 AGN中观测到的垂直偏振水平，环形结构的半开角必须大于60°，表明大多数赛克特2型AGN具有宽广、开放的环形结构。
• 具有τ ≈ 1的极向喷流可在类型2视线方向产生垂直偏振，但前提是其光学深度不过高（τ < 1），以维持偏振二分性。
• 类型2 AGN的偏振度对风的光学深度不敏感，但当τ > 0.3时，会显著改变偏振角分布，从而影响类型1/类型2偏振二分性。
• 当引入赤道散射时，模型预测偏振谱呈现更明显的凹形特征，为探测在外观上缺乏宽线区的AGN中是否存在此类区域提供了诊断工具。