arXiv 0711.21132007-11-14astro-ph

吸积盘与AGN连续统变率的性质和起源

Accretion Disks and the Nature and Origin of AGN Continuum Variability

C. Martin Gaskell

备注：: Invited talk given at "The Nuclear Region, Host Galaxy, and Environment of Active Galaxies", Huatulco, Mexico, April 2007. To appear in Rev. Mex. A&A Conf. Ser. 11 pages, 7 figures

摘要：

研究活动星系核中占主导地位的热连续辐射的理论和观测。经红化校正后，活动星系核的稳态光-紫外光谱能量分布（SED）非常相似。SED在能量上主要由蓝色大凸起（BBB）控制，但这个凸起从未显示出对于具有r^{-0.75}径向温度梯度的标准薄吸积盘所预测的nu^{+1/3}光谱。相反，观察到的光学紫外SED意味着温度梯度r^{-0.57}与盘的厚度无关。这意味着在圆盘中有一些热量向外流动。盘是大的，并且发射光学连续谱的区域与内部宽线区域（BLR）一样大。因为在所有活动星系核中，光变都出现在BLR的光交叉时间上，所以光变必须以接近光速的速度传播，而不是在动力学时间尺度上传播。这表明能量产生机制是电磁的，而不是流体动力的。由于速度接近光速，因此在发射中可能存在显著的局部各向异性。BBB的大的快速变化意味着磁流体动力学能量产生是根本不稳定的。由于吸积物质中不可避免的径向温度梯度，不同的光谱区域主要来自不同的半径，并且不同光谱区域中的变化对应于不同半径处的变化。这解释了经常观察到的X射线和光学变化的独立性，在较低能量下的变化导致在较高能量下的变化，以及发射线混响滞后的快速变化。提出了局部变率假设的一些观测检验。

英文摘要：

Theory and observations of the dominant thermal continuum emission in AGNs are examined. After correction for reddening, the steady state AGN optical--UV spectral energy distributions (SEDs) are very similar. The SEDs are dominated energetically by the big blue bump (BBB), but this bump never shows the nu^{+1/3} spectrum predicted for a standard thin accretion disk with a r^{-0.75} radial temperature gradient. Instead, the observed optical-UV SED implies a temperature gradient of r^{-0.57} independent of the thickness of the disk. This means that there is some flow of heat outwards in the disk. The disk is large and the region emitting the optical continuum is as large as the inner broad-line region (BLR). Because optical variability is seen in all AGNs on the light-crossing time of the BLR, variations must propagate at close to the speed of light, rather than on dynamical timescales. This argues that the energy-generation mechanism is electromagnetic rather that hydrodynamic. Since the velocities are near the speed of light, there can be significant local anisotropy in the emission. The large rapid variations of the BBB imply that the magnetohydrodynamic energy generation is fundamentally unstable. Because of the inevitable radial temperature gradient in the accreting material, different spectral regions come predominantly from different radii, and variations in different spectral regions correspond to variability at different radii. This explains the frequently observed independence of X-ray and optical variations, cases of variability at lower energies leading variability at higher energies, and rapid changes in emission-line reverberation lags. Some observational tests of the local variability hypothesis are proposed.

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摘要：

英文摘要：

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