Thermal Instability and Photoionized X-Ray Reflection in Accretion Disks
Thermal Instability and Photoionized X-Ray Reflection in Accretion Disks
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吸积盘中的热不稳定性和光电离 X 射线反射
DOI：
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发表时间：
1999
期刊：
影响因子：
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通讯作者：
T. Kallman
中科院分区：
文献类型：
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作者：
S. Nayakshin;D. Kazanas;T. Kallman
我们研究了致密天体附近的吸积盘被覆盖的x射线源照射的情况。我们的方法不同于以往的研究，因为我们放宽了这些研究中使用的恒定气体密度的简化假设；相反，我们从流体静力平衡中确定密度，该平衡与电离平衡和辐射传递在平面平行几何中同时求解。我们计算了一系列物理条件下被照射层的温度分布和再处理的x射线光谱，照明辐射的光子指数Γ值，以及入射和视角。根据一些早期的研究，我们发现自洽密度测定表明了在类星体发射线研究中众所周知的热电离不稳定性的存在。这种不稳定性的主要作用是防止被照射的气体达到对热扰动不稳定的温度。因此，与预测照明材料中气体温度连续且相当平滑变化的恒定密度计算形成鲜明对比的是，我们发现温度剖面由几个定义良好的热稳定层组成。这些稳定层之间的过渡非常尖锐，就再处理光谱而言，可以视为不连续。特别是，x射线照射气体的最上层几乎完全电离，处于局部康普顿温度（~107- 108k）；在较大的深度，气体温度急剧下降，形成T ~ 106 K的薄层，而在更大的深度，气体温度急剧下降到盘的有效温度。对于给定的x射线光谱指数，这种不连续的温度结构仅由一个参数a控制，a表示相对于入射x射线通量的引力强度。我们发现大多数铁Kα线的发射和吸收边产生于最冷、最深的层，而最热、最上层的铁原子通常几乎完全电离，因此在~6.4-10 keV能量范围内对再加工特征的贡献可以忽略不计。我们还发现，顶部热层的汤姆逊深度对于确定穿透到较深较冷层的x射线通量的比例至关重要，从而直接影响铁线、边缘和反射特征的强度。由于这些效应的相互作用，在Γ > 2时，铁特征的等效宽度（EW）随光照通量的大小而单调减小，而线质心能量保持在6.4 keV。总结了x射线反射光谱中再处理特征与问题的重力参数a、光谱指数Γ等参数的依赖关系。我们强调，我们的自洽计算结果在数量和质量上都不同于使用恒定密度假设获得的结果。因此，我们建议未来的x射线反射计算应该始终使用流体静力平衡，以便为活动星系核和星系黑洞候选体的x射线光谱提供可靠的解释。
We study the illumination of accretion disks in the vicinity of compact objects by an overlying X-ray source. Our approach differs from previous works of the subject in that we relax the simplifying assumption of constant gas density used in these studies; instead we determine the density from hydrostatic balance which is solved simultaneously with the ionization balance and the radiative transfer in a plane-parallel geometry. We calculate the temperature profile of the illuminated layer and the reprocessed X-ray spectra for a range of physical conditions, values of photon index Γ for the illuminating radiation, and the incident and viewing angles. In accordance with some earlier studies, we find that the self-consistent density determination makes evident the presence of a thermal ionization instability well known in the context of quasar emission line studies. The main effect of this instability is to prevent the illuminated gas from attaining temperatures at which the gas is unstable to thermal perturbations. Thus, in sharp contrast to the constant density calculations that predict a continuous and rather smooth variation of the gas temperature in the illuminated material, we find that the temperature profile consists of several well defined thermally stable layers. Transitions between these stable layers are very sharp and can be treated as discontinuities as far as the reprocessed spectra are concerned. In particular, the uppermost layers of the X-ray illuminated gas are found to be almost completely ionized and at the local Compton temperature (~107-108 K); at larger depths, the gas temperature drops abruptly to form a thin layer with T ~ 106 K, while at yet larger depths it decreases sharply to the disk effective temperature. For a given X-ray spectral index, this discontinuous temperature structure is governed by just one parameter, A, which characterizes the strength of the gravitational force relative to the incident X-ray flux. We find that most of the Fe Kα line emission and absorption edge are produced in the coolest, deepest layers, while the Fe atoms in the hottest, uppermost layers are generally almost fully ionized, hence making a negligible contribution to reprocessing features in the ~6.4-10 keV energy range. We also find that the Thomson depth of the top hot layers is pivotal in determining the fraction of the X-ray flux which penetrates to the deeper cooler layers, thereby affecting directly the strength of the Fe line, edge and reflection features. Due to the interplay of these effects, for Γ ≲ 2, the equivalent width (EW) of the Fe features decreases monotonically with the magnitude of the illuminating flux, while the line centroid energy remains at 6.4 keV. We provide a summary of the dependence of the reprocessing features in the X-ray reflected spectra on the gravity parameter A, the spectral index Γ, and other parameters of the problem. We emphasis that the results of our self-consistent calculations are both quantitatively and qualitatively different from those obtained using the constant density assumption. Therefore, we propose that future X-ray reflection calculations should always utilize hydrostatic balance in order to provide a reliable interpretation of X-ray spectra of active galactic nuclei and galactic black hole candidates.