【作者】 甘朝明；

【导师】 汪定雄；

【作者基本信息】 华中科技大学 ， 理论物理， 2010， 博士

【摘要】 黑洞是广义相对论的一个重要的预言。现在人们已经普遍相信,几乎在每一个星系中央都存在着一个超大质量黑洞。以黑洞为核心的致密天体,包括活动星系核和某些X射线双星、伽玛射线暴等,展现出了一些非常极端的高能辐射现象,并时常伴随着猛烈的喷流和复杂的光变。黑洞吸积是这些致密天体“中心发动机”事实上的标准模型。经过几十年的发展,吸积盘理论日趋完善,但也存在着一些基本的问题尚无定论,如粘滞的物理过程、盘上外流的发生机制等。近年来,人们逐渐意识到磁场可能在物质吸积的过程中扮演着非常重要的角色,其中包括小尺度和大尺度两种形态的磁场。前者对应于吸积流内部的粘滞过程以促使吸积的进行,如“磁旋转不稳定性”(MRI)；后者则被认为与喷流的加速、准直以及宏观的能量、角动量转移相关,如Blandford-Payne(BP)过程。除吸积外,快速旋转的黑洞其本身也是一个巨大的能源。在本文中我们详细地阐述了通过大尺度磁场从黑洞提取能量的物理机制,包括Blandford-Znajek (BZ)过程和磁耦合(Magnetic Coupling, MC)过程。本文细致地论述了MC过程中的能量转移及其在盘内部耗散的过程、MC过程对吸积盘辐射特征的影响、MC过程对吸积盘位形的影响——由于MC过程导致的盘内边缘位置的移动。考虑到大尺度闭合磁场对等离子体的束缚作用,我们提出了一个“磁化吸积盘—冕”模型。其中,MC过程的大尺度磁场很自然地为冕物质提供了一个落向黑洞视界面的通道,籍此我们得以限制冕的分布。通过综合求解黑洞与吸积盘、吸积盘与冕之间的能量传递及其对盘动力学和磁场分布的影响,我们自洽地得到了盘冕系统的整体解,并用Monte-Carlo方法模拟了系统的出射谱。结果表明,盘冕系统很自然地具有“高光度、软谱态”或者“低光度、硬谱态”的辐射特征,这一特性可用于解释一些黑洞双星的高能X射线非热谱。另外,越来越多的观测证据表明,喷流与吸积之间存在着密切的联系。我们从能量和角动量平衡的角度出发论述了磁化外流与吸积盘之间的耦合。盘上发出的外流将降低吸积盘的光度,而吸积盘的动力学性质反过来也限制着对外流的功率输出。籍此我们解释了一些射电类星体盘光度与喷流功率的相关关系。另外,考虑到外流可以有效地带走角动量,我们讨论了通过磁化外流解决吸积盘角动量整体平衡的可能性。在我们的模型中,黑洞自转的快慢和大尺度磁场位形至关重要。最后我们对这两个问题进行了讨论,并期望日后能够找到一些观测上的判别。作为攻读博士学位期间的学习和工作总结,结合自己的理解本文引述了较多的理论背景知识。作者本人的工作主要分布在第2.3、2.4、3.2、4.2、4.3和5.1节,感兴趣的读者可直接跳读到相关的章节。更多还原

【Abstract】 The existence of black holes is an important prediction of general relativity. It is widely believed that a super massive black holes lies in the center of every galaxy. And some extremely high-energy phenomena have been observed in some black-hole systems, including Active Galactic Nuclei and some X-Ray Binaries and Gamma-Ray Bursts.Black-hole accretion is the de facto standard model for the central engines of those compact objects. The accretion theory is well established, but there are still some essential problems, e.g., the physical viscosity process, the mechanism for launching outflow from disks. Recently, it is realized that both small-scale and large-scale magnetic fields might play important roles in the accretion physics. The small-scale magnetic fields are related closely to the internal viscosity which promotes accretion. And the larger-scale magnetic fields can give rise to "large-scale" transfers of energy and angular momentum, e.g., being in charge of accelerating and collimating jets.The rotation energy of a fast spinning black hole is also extractable. We discuss two main methods of extracting energy from a spinning black hole, i.e., Blandford-Znajeck (BZ) process and Magnetic Coupling (MC) process. The following aspects are investigated in details:(ⅰ) the energy budget and dissipation in the MC process, (ⅱ) the MC effects on disk radiation; (ⅲ) the derivation of inner edge of a thin disk from the innermost stable circular orbit due to the MC effects.We propose a magnetically induced disk-corona model, in which the closed field lines of MC process provide natural channels for corona matter falling onto the central black hole. We derive the boundaries of corona according to the configuration of large-scale magnetic fields. We obtained the global solution of the system numerically, taking into account the energy transfer among black hole, disk and corona and also its effects on the disk dynamics. Finally, the emerged spectra are simulated by using Monte-Carlo method. It turns out the disk-corona system naturally gives rise to hard spectra with low luminosity or soft spectra with high luminosity. This property could be used to explain the non-thermal hard X-ray spectra observed in black hole binaries.There are many observational implications for disk-jet connection. We discuss the coupling between magnetic outflow and disk accretion based on the balance of energy and angular momentum, trying to explain the correlation between the disk luminosity and jet power observed in dozens of radio quasars. We also discussed the contribution of magnetic outflow to the overall equilibrium of angular momentum in accretion disk.There are two key points related to our model, i.e., black hole spin and the configuration of large-scale magnetic fields. We give a general comment on these issues, expecting to do some new works based on the observations in future.更多还原