Numerical investigations of the inner regions of black hole accretion discs

黑洞吸积盘内部区域的数值研究

• 批准号: 2738304
• 金额: --
• 依托单位: University of Oxford
• 依托单位国家: 英国
• 项目类别: Studentship
• 财政年份: 2023
• 资助国家: 英国
• 起止时间: 2023 至 无数据
• 项目状态: 未结题
• 来源: https://gtr.ukri.org/projects?ref=studentship-2738304
• 关键词: Numerical; investigations; inner; regions; black

Black hole accretion studies have history spanning some 54 years, going back to Donald Lynden-Bell's seminal paper (1969, Nature, 223, 690) in which he put forth the case that the then deeply mysterious quasars and active galactic nuclei were being powered by large black holes accreting gas from their surroundings in the form of a disc. In the ensuing half century, the field of black hole accretion disc studies has grown enormously on many fronts: observational, phenomenological modelling, numerical simulations and fundamental MHD processes. In particular, a steady-state model of accretion discs developed by Shakura and Sunyaev (1973, AA, 24, 347) has become a bedrock standard for both theory and observations. It combines both dynamical and radiation physics. Because of Shakura-Sunyaev theory applies to "thin discs," in which the vertical scale height is much less than the radial extent of interest, direct numerical study of such (turbulent) discs has been very difficult. In fact, the theory itself has never been verified directly, even though it is widely used. We are now in a position where this has become possible, in no small part because of the availability of state-of-the-art hardware and code development. In this thesis, which is designed to be a detailed study of the accretion flow near the innermost edge of black hole accretion discs, we will set up a suite of test problems to explicitly test Shakura-Sunyaev theory as well to explore more generally how the theory breaks down, as we know it must at some point. The observational setting in which this will be done is the rapidly growing field of tidal disruption events, or TDEs. This is an event that occurs when a star approaches a massive black hole at the centre of its galaxy in such a way that it is tidally torn apart. While some stellar debris invariably escapes, some also remains behind, and in its later evolutionary stages forms a time-dependent accretion disc whose emission is potentially ripe with information about the central black hole. The computational tool that will be used to study this problem is the Athena ++ code, developed by Prof James Stone and his co-workers at the IAS in Princeton. The first part of this thesis will be to use Athena ++ in its full radiative mode to study time dependent disc accretion in a set of controlled problems. The code is remarkable in that it is fully relativistic, MHD, and allows radiation to be included self-consistently not just in a post-processing sense. We benefit greatly from the additional presence of Prof Stone on this project, who has agreed to become involved at a hands-on level, so that technical details of setting up and running the code on the appropriate cluster can be done with maximum efficiency. The impact of this work promises to be enormous, bearing upon both fundamental disc theory and observation, the disc spectrum being an integral part of the calculation itself. The primary application will be to the light curves of TDEs. This includes such problems as the Lightman-Eardley Instability, a coupling between the disc turbulence and the radiative heating whose role in disc evolution is still not fully understood. This study has the numerical resolution and radiation physics capabilities to investigate this problem at a far higher level than previous studies. We shall also investigate the effect of magnetic field geometry, especially its role in setting the overall evolutionary behaviour of the disc, which is important for understanding when an MHD jet emerges from the inner regions and when it does not. A second major part of this thesis will be the investigation of the flow within what is normally considered the inner edge of the disc. Black hole discs extend down toward smaller and smaller radii until at some point, still well outside of the event horizon, the circular orbits become unstable. This transition radius is known as the innermost stable

黑洞吸积研究已有大约 54 年的历史，可以追溯到 Donald Lynden-Bell 的开创性论文（1969 年，Nature，223, 690），其中他提出了当时非常神秘的类星体和活动星系核的动力来自于大黑洞以圆盘的形式从周围吸积气体。在接下来的半个世纪中，黑洞吸积盘研究领域在许多方面都取得了巨大的发展：观测、现象学建模、数值模拟和基本 MHD 过程。特别是，Shakura 和 Sunyaev (1973, AA, 24, 347) 开发的吸积盘稳态模型已成为理论和观测的基石标准。它结合了动力学和辐射物理学。由于 Shakura-Sunyaev 理论适用于“薄圆盘”，其中垂直尺度高度远小于感兴趣的径向范围，因此对此类（湍流）圆盘进行直接数值研究非常困难。事实上，尽管该理论被广泛使用，但其本身从未得到直接验证。我们现在处于这样的位置，这已经成为可能，这在很大程度上是因为拥有最先进的硬件和代码开发。在本论文中，旨在详细研究黑洞吸积盘最内边缘附近的吸积流，我们将设置一套测试问题来明确测试 Shakura-Sunyaev 理论，并更普遍地探索如何理论会崩溃，正如我们所知，它一定会在某个时刻崩溃。进行这项工作的观测环境是快速增长的潮汐破坏事件（TDE）领域。当一颗恒星接近其星系中心的一个大质量黑洞并被潮汐撕裂时，就会发生这种事件。虽然一些恒星碎片总是会逃逸，但有些仍然留在后面，在其后期的演化阶段形成一个依赖于时间的吸积盘，其发射可能已经成熟，其中包含有关中心黑洞的信息。用于研究这个问题的计算工具是 Athena ++ 代码，由 James Stone 教授和他在普林斯顿 IAS 的同事开发。本论文的第一部分将使用 Athena ++ 的全辐射模式来研究一组受控问题中的时间依赖性椎间盘增生。该代码的非凡之处在于它是完全相对论的、MHD 的，并且允许辐射自洽地包含在内，而不仅仅是在后处理意义上。 Stone 教授参与该项目让我们受益匪浅，他同意亲身参与，以便能够以最高效率完成在适当集群上设置和运行代码的技术细节。这项工作的影响是巨大的，对基本的圆盘理论和观测都有影响，圆盘谱是计算本身不可分割的一部分。主要应用是 TDE 的光变曲线。这包括诸如莱特曼-埃尔德利不稳定性等问题，这是盘湍流和辐射加热之间的耦合，其在盘演化中的作用尚未完全了解。这项研究具有比以前的研究更高水平的数值分辨率和辐射物理能力来研究这个问题。我们还将研究磁场几何形状的影响，特别是它在设定圆盘整体演化行为中的作用，这对于理解 MHD 射流何时从内部区域出现以及何时不出现非常重要。本论文的第二个主要部分是研究通常被认为是圆盘内边缘的流动。黑洞盘向下延伸到越来越小的半径，直到在某个点上，仍在事件视界之外，圆形轨道变得不稳定。该过渡半径被称为最内稳定半径