Polarized image of equatorial emission in the Kerr geometry
Polarized image of equatorial emission in the Kerr geometry
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DOI：
10.1103/physrevd.104.044060
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发表时间：
2021-05
影响因子：
5
通讯作者：
Z. Gelles;E. Himwich;Michael D. Johnson;D. Palumbo
中科院分区：
文献类型：
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作者：
Z. Gelles;E. Himwich;Michael D. Johnson;D. Palumbo
利用零测地线方程的半解析解和彭罗斯-沃克常数守恒，我们建立了一个简单的玩具模型，用于计算克尔黑洞周围赤道源同步辐射的偏振图像。我们的模型是Narayan等人的扩展。(2021)，它给出了Schwarzschild黑洞的结果，包括完全解析近似。我们的模型包括任意的观测者倾角、黑洞自转、局域增强和局域磁场构型。我们研究了黑洞自旋对光子平行输运的几何效应，并将这些效应从发射区相对论、引力和电磁过程的复杂组合中分离出来。我们得到了直接像中自旋对观测到的面对面极化旋转的次前导几何效应的解析近似：$\rM EVPA}\sim-2a/r_{\rM S}^2$，其中$a$是黑洞自旋，$r_{\rM S}$是发射半径。我们进一步证明了自旋在面子图像上引入了有序统一效应：$\Delta{\rm EVPA}\sim\pm a/\sqrt{27}$。我们还使用我们的玩具模型来分析在轨道热点耀斑期间观察到的偏振“环”。我们的模型提供了对黑洞的极化模拟和观测的洞察力，例如由EHT和引力所做的那些。
We develop a simple toy model for polarized images of synchrotron emission from an equatorial source around a Kerr black hole by using a semi-analytic solution of the null geodesic equation and conservation of the Penrose-Walker constant. Our model is an extension of Narayan et al. (2021), which presented results for a Schwarzschild black hole, including a fully analytic approximation. Our model includes an arbitrary observer inclination, black hole spin, local boost, and local magnetic field configuration. We study the geometric effects of black hole spin on photon parallel transport and isolate these effects from the complicated combination of relativistic, gravitational, and electromagnetic processes in the emission region. We find an analytic approximation, consistent with previous work, for the subleading geometric effect of spin on observed face-on polarization rotation in the direct image: $\Delta {\rm EVPA} \sim -2a/r_{\rm s}^2$, where $a$ is the black hole spin and $r_{\rm s}$ is the emission radius. We further show that spin introduces an order unity effect on face-on subimages: $\Delta {\rm EVPA} \sim \pm a/\sqrt{27}$. We also use our toy model to analyze polarization"loops"observed during flares of orbiting hotspots. Our model provides insight into polarimetric simulations and observations of black holes such as those made by the EHT and GRAVITY.