Polarized Anisotropic Synchrotron Emission and Absorption and Its Application to Black Hole Imaging

IF 4.8 2区 物理与天体物理 Q1 ASTRONOMY & ASTROPHYSICS
Alisa Galishnikova, Alexander Philippov, Eliot Quataert
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Abstract

Abstract Low-collisionality plasma in a magnetic field generically develops anisotropy in its distribution function with respect to the magnetic field direction. Motivated by the application to radiation from accretion flows and jets, we explore the effect of temperature anisotropy on synchrotron emission. We derive analytically and provide numerical fits for the polarized synchrotron emission and absorption coefficients for a relativistic bi-Maxwellian plasma (we do not consider Faraday conversion/rotation). Temperature anisotropy can significantly change how the synchrotron emission and absorption coefficients depend on observing angle with respect to the magnetic field. The emitted linear polarization fraction does not depend strongly on anisotropy, while the emitted circular polarization does. We apply our results to black hole imaging of Sgr A* and M87* by ray tracing a GRMHD simulation and assuming that the plasma temperature anisotropy is set by the thresholds of kinetic-scale anisotropy-driven instabilities. We find that the azimuthal asymmetry of the 230 GHz images can change by up to a factor of 3, accentuating ( T ⊥ > T ∥ ) or counteracting ( T ⊥ < T ∥ ) the image asymmetry produced by Doppler beaming. This can change the physical inferences from observations relative to models with an isotropic distribution function, e.g., by allowing for larger inclination between the line of sight and spin direction in Sgr A*. The observed image diameter and the size of the black hole shadow can also vary significantly due to plasma temperature anisotropy. We describe how the anisotropy of the plasma can affect future multifrequency and photon ring observations. We also calculate kinetic anisotropy-driven instabilities (mirror, whistler, and firehose) for relativistically hot plasmas.
偏振各向异性同步加速器发射和吸收及其在黑洞成像中的应用
摘要低碰撞等离子体在磁场中的分布函数一般随磁场方向呈各向异性。以吸积流和喷流辐射为研究对象,探讨了温度各向异性对同步辐射的影响。我们对相对论双麦克斯韦等离子体的偏振同步辐射和吸收系数(我们不考虑法拉第转换/旋转)进行了解析推导并提供了数值拟合。温度各向异性可以显著改变同步辐射和吸收系数随观测角度的变化。发射的线偏振分数与各向异性关系不大,而圆偏振分数与各向异性关系很大。我们通过射线追踪GRMHD模拟将结果应用于Sgr A*和M87*的黑洞成像,并假设等离子体温度各向异性由动力学尺度各向异性驱动的不稳定性阈值设定。我们发现,230 GHz图像的方位角不对称性最多可以改变3倍,从而使(T⊥>T∥)或抵消(T⊥<T∥)多普勒光束产生的图像不对称性。这可以改变从观测到具有各向同性分布函数的模型的物理推断,例如,允许在Sgr A*的视线和旋转方向之间有更大的倾斜。由于等离子体温度的各向异性,观测到的图像直径和黑洞阴影的大小也会发生很大的变化。我们描述了等离子体的各向异性如何影响未来的多频和光子环观测。我们还计算了相对热等离子体的动力学各向异性驱动的不稳定性(镜面,哨声和消防软管)。
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来源期刊
Astrophysical Journal
Astrophysical Journal 地学天文-天文与天体物理
CiteScore
8.40
自引率
30.60%
发文量
2854
审稿时长
1 months
期刊介绍: The Astrophysical Journal is the foremost research journal in the world devoted to recent developments, discoveries, and theories in astronomy and astrophysics.
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