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arXiv 2208.02269astro-ph.HE
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辐射无效吸积流中的磁通量输运和磁滞盘的路径

Magnetic Flux Transport in Radiatively Inefficient Accretion Flows and the Pathway towards a Magnetically Arrested Disk

Prasun Dhang, Xue-Ning Bai, Christopher J. White

摘要:

大尺度磁场在决定角动量输运和产生吸积系统中的喷流/外流方面起着至关重要的作用,但其起源仍然知之甚少。我们专注于辐射低效吸积流(RIAF)周围的黑洞(BH),并进行三维广义相对论磁流体动力学(GRMHD)模拟使用雅典娜++代码。我们首先重新确认,MRI发电机在RIAF单独不会自发形成磁制动盘(MAD),有利于强射流的形成。我们接下来研究另一种可能性,即大尺度磁场是从外部源(例如X射线双星中的伴星,活动星系核中的磁化环境介质)平流向内的。虽然外部磁场的实际结构可能复杂而不确定,但它们很可能是封闭的。作为第一个研究,我们把它们作为不同大小,形状和场强的闭合场回路。与早期的磁通量运输的研究,其中磁通量被注入在初始层流中,我们注入的磁场环路中的准稳态湍流RIAF流入平衡,并遵循其演变。我们发现,一个相当大的分数($\sim15\%-40\%$)的磁通量注入在大半径到达BH与弱依赖的环路参数,除了当环路被注入在高纬度,远离中平面。在我们的研究中观察到的相对高效率的通量传输暗示,MAD可能会形成相对容易接近BH,提供了一个源的大尺度场存在于较大的半径。

英文摘要:

Large-scale magnetic fields play a vital role in determining the angular momentum transport and generating jets/outflows in the accreting systems, yet their origin remains poorly understood. We focus on radiatively inefficient accretion flows (RIAFs) around the black holes (BHs), and conduct three-dimensional general-relativistic magnetohydrodynamic (GRMHD) simulations using the Athena++ code. We first re-confirm that the MRI dynamo in the RIAF alone does not spontaneously form a magnetically arrested disk (MAD), conducive for strong jet formation. We next investigate the other possibility, where the large-scale magnetic fields are advected inward from external sources (e.g. the companion star in X-ray binaries, magnetized ambient medium in AGNs). Although the actual configuration of the external fields could be complex and uncertain, they are likely to be closed. As a first study, we treat them as closed field loops of different sizes, shapes and field strengths. Unlike earlier studies of flux transport, where magnetic flux is injected in the initial laminar flow, we injected the magnetic field loops in the quasi-stationary turbulent RIAF in inflow equilibrium and followed their evolution. We found that a substantial fraction ($\sim15\%-40\%$) of the flux injected at the large radii reaches the BH with a weak dependence on the loop parameters except when the loops are injected at high latitudes, away from the mid-plane. Relatively high efficiency of flux transport observed in our study hints that a MAD might be formed relatively easily close to the BH, provided that a source of the large-scale field exists at the larger radii.

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