发表机构
JILA, University of Colorado and National Institute of Standards and Technology; Department of Astrophysical Sciences, Princeton University; Princeton Plasma Physics Laboratory; Department of Astrophysical and Planetary Sciences, University of Colorado（科罗拉多大学JILA与美国国家标准与技术研究院; 普林斯顿大学天体科学系; 普林斯顿等离子体物理实验室; 科罗拉多大学天体与行星科学系）
机构由 AI 辅助整理，请以论文原文为准。AI 中文总结
本研究结合动理学与流体模型，发现微观等离子体不稳定性引起的弱散射导致显著粘性阻尼，从而抑制无碰撞吸积流中的螺旋密度波，降低发射区变率并影响EHT观测解读。
AI 中文摘要
螺旋密度波是差动旋转流中普遍存在的现象。它们是非模态、非轴对称的扰动，由于背景流中的剪切作用，其波数、频率和振幅随时间演化。在对光学薄、低光度黑洞吸积流的全局模拟中，这类波的激发导致等离子体密度中出现螺旋图案，从而影响发射区的形态。因此，此类吸积流的观测变异性预计会受到螺旋密度波行为的影响。迄今为止，对吸积流中螺旋密度波的研究均假设吸积流等离子体始终处于局部热力学平衡（LTE）状态。实际上，热且稀薄的吸积流等离子体预计会远离LTE，应相应地进行建模。在此，我们结合动理学模型和流体模型来描述无碰撞剪切片中的螺旋密度波传播、阻尼和激发，这比假设LTE的模型更准确地代表了低光度吸积流的等离子体条件。我们发现，微观等离子体不稳定性在决定螺旋密度波行为方面尤为重要，因为它们引起的弱粒子散射导致显著的粘性阻尼。这些结果的观测意义包括发射区变率降低，这可能改善事件视界望远镜（EHT）观测与模拟/理论之间的一致性，以及相比采用LTE的模拟，发射区图案速度降低，这对最终EHT影片的解读具有影响。
英文摘要
Spiral density waves are ubiquitous phenomena in differentially rotating flows. They are non-modal, non-axisymmetric perturbations that, due to shear in the background flow, possess time-evolving wavenumbers, frequencies, and amplitudes. In global simulations of optically thin, low-luminosity black-hole accretion flows, the excitation of such waves leads to spiral patterns in the plasma density that influence the morphology of the emission region. As such, the observed variabilities of such accretion flows are expected to be affected by the behavior of spiral density waves. Thus far, investigations of spiral density waves in accretion flows have exclusively used the assumption that the accretion-flow plasmas are in local thermodynamic equilibrium (LTE) at all times. In reality, hot and diffuse accretion-flow plasmas are expected to deviate far from LTE, and should be modeled accordingly. Here, we employ a combination of kinetic and fluid models to describe spiral density wave propagation, damping, and excitation in a collisionless shearing sheet, which more accurately represents the plasma conditions of low-luminosity accretion flows than do models that assume LTE. We find that micro-physical plasma instabilities are especially important in determining how spiral density waves behave, because the weak particle scattering that they induce leads to appreciable viscous damping. Observational implications of these results include reduced emission-region variability, which may improve agreement between Event Horizon Telescope (EHT) observations and simulation/theory, as well as a reduction in emission-region pattern speeds compared to simulations employing LTE, which bears on the interpretation of eventual EHT movies.
Comments23 pages, 9 figures. Submitted to MNRAS. Comments welcome!
