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arXiv 2012.06504astro-ph.HE
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黑洞吸积盘辐射的光谱和偏振特性:包括吸收效应

Spectral and polarization properties of black hole accretion disc emission: including absorption effects

Roberto Taverna, Lorenzo Marra, Stefano Bianchi, Mičhal Dovciak, René Goosmann, Frederic Marin, Giorgio Matt, Wenda Zhang

摘要:

研究从黑洞吸积盘发出的辐射是理解这些源的主要物理性质,特别是黑洞自旋的重要途径。除了光谱分析之外,偏振测量也变得越来越重要,这是由于新技术的发展,这些新技术很快将允许在X射线和γ射线中进行测量。从软态的黑洞吸积盘发射的光子确实被期望被极化,其能量依赖性可以提供对黑洞自旋的估计。然而,到目前为止进行的计算认为散射是确定所发射辐射的偏振状态的唯一过程,隐含地假设所涉及的温度是这样的,即圆盘中的材料完全电离。在这项工作中,我们概括的问题,通过计算的电离结构的表面层的光盘与公共代码云,然后通过确定新兴的辐射的偏振特性,使用蒙特卡罗代码斯托克斯。这使我们能够考虑吸收效应和散射效应。我们表明,包括吸收可以深刻地修改相对于什么是在纯散射限制所获得的新兴辐射的偏振特性。作为一般规则,我们发现,当吸收更重要时,偏振度更大,例如,当最里面的吸积盘区域中的物质的电离远未完成时,发生低吸积率和/或自旋。

英文摘要:

The study of radiation emitted from black hole accretion discs represents a crucial way to understand the main physical properties of these sources, and in particular the black hole spin. Beside spectral analysis, polarimetry is becoming more and more important, motivated by the development of new techniques which will soon allow to perform measurements also in the X- and γ-rays. Photons emitted from black hole accretion discs in the soft state are indeed expected to be polarized, with an energy dependence which can provide an estimate of the black hole spin. Calculations performed so far, however, considered scattering as the only process to determine the polarization state of the emitted radiation, implicitly assuming that the temperatures involved are such that material in the disc is entirely ionized. In this work we generalize the problem by calculating the ionization structure of a surface layer of the disc with the public code CLOUDY , and then by determining the polarization properties of the emerging radiation using the Monte Carlo code STOKES . This allows us to account for absorption effects alongside scattering ones. We show that including absorption can deeply modify the polarization properties of the emerging radiation with respect to what is obtained in the pure-scattering limit. As a general rule, we find that the polarization degree is larger when absorption is more important, which occurs e.g. for low accretion rates and/or spins when the ionization of the matter in the innermost accretion disc regions is far from complete.

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