arXivDaily arXiv每日学术速递 周一至周五更新
arXiv 1709.09799astro-ph.HE
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活动星系核扩展吸积日冕的中心集中X射线辐射

Centrally Concentrated X-ray Radiation from an Extended Accreting Corona in Active Galactic Nuclei

B. F. Liu, Ronald E. Taam, Erlin Qiao, Weimin Yuan

摘要:

活动星系核的X射线发射被认为是起源于位于一个冷的、几何薄的吸积盘上方的热日冕。从观测中推断出一个高度集中的日冕位于黑洞上方的$\sim10$引力半径内。基于星际介质/风的吸积,提出了一个盘状日冕模型,其中日冕通过辐射、热传导以及质量交换\citep{Liu2015, Qiao2017}与盘状体良好耦合。这样的模型避免了人工能量输入的日冕,并已被用来解释在活动星系核中观察到的光谱特征。在这项工作中,它表明,在我们的模型中的扩展吸积流的日冕体发射的尺寸是非常小的。超过80%的硬X射线能量是从一个非旋转的黑洞周围10个史瓦西半径内的小区域发射的,对于旋转的黑洞,这个区域预计会更小。在这里,电晕发射在更高的爱丁顿比下更扩展。的紧凑性参数的电晕,$l={L\over R}{σ_{\rm T}\over m_{\rm e} c^3}$,被证明是在1-33的范围内的爱丁顿比为0.02 - 0.1。结合日冕中的电子温度,这表明电子-正电子对的产生在这一制度中并不占主导地位。紧度参数与光子指数之间存在正相关关系。通过将上述模型的预测结果与观测特征进行比较,我们发现模型与观测结果是一致的。

英文摘要:

The X-ray emission from bright active galactic nuclei (AGNs) is believed to originate in a hot corona lying above a cold, geometrically thin accretion disk. A highly concentrated corona located within $\sim10$ gravitational radii above the black hole is inferred from observations. Based on the accretion of interstellar medium/wind, a disk corona model has been proposed in which the corona is well coupled to the disk by radiation, thermal conduction, as well as by mass exchange \citep{Liu2015, Qiao2017}. Such a model avoids artificial energy input to the corona and has been used to interpret the spectral features observed in AGN. In this work, it is shown that the bulk emission size of the corona is very small for the extended accretion flow in our model. More than 80\% of the hard X-ray power is emitted from a small region confined within 10 Schwarzschild radii around a non-spinning black hole, which is expected to be even smaller accordingly for a spinning black hole. Here, the corona emission is more extended at higher Eddington ratios. The compactness parameter of the corona, $l={L\over R}{σ_{\rm T}\over m_{\rm e} c^3}$, is shown to be in the range of 1-33 for Eddington ratios of 0.02 - 0.1. Combined with the electron temperature in the corona, this indicates that electron--positron pair production is not dominant in this regime. A positive relation between the compactness parameter and photon index is also predicted. By comparing the above model predictions with observational features, we find that the model is in agreement with observations.

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