Emission Line Profiles and Images of Geometrically Thin and Optically Thick Accretion Disks

Under the assumption that the accretion disk around a Kerr (spinning) black hole is geometrically thin and optically thick, the trajectories of photons in Kerr metric are calculated by using the photon tracing method. And by numerical calculations, we have made a study on the relativistic iron line profiles and images of thin accretion disks. The result shows that viewing at large inclination angles, because of the contribution of the photons from the lower surface of the accretion disk, the line profile becomes double-peaked and the flux image is also significantly modified.     

几何薄光学厚吸积盘谱线轮廓及成像研究

假设位于黑洞赤道面上做圆形轨道运动的吸积盘是几何薄、光学厚的.利用光子追踪法计算在Kerr度规下的光子运动轨迹,通过数值计算研究薄吸积盘的相对论谱线轮廓及成像.在大角度观测时,吸积盘下表面的光子对谱线轮廓及成像的影响是显著的.     

Relativistic Jets from Accretion Disks

The jets observed to emanate from many compact accreting objects may arise from the twisting of a magnetic field threading a differentially rotating accretion disk which acts to magnetically extract angular momentum and energy from the disk. Two main regimes have been discussed, hydromagnetic jets, which have a significant mass flux and have energy and angular momentum carried by both matter and electromagnetic field and, Poynting jets, where the mass flux is small and energy and angular momentum are carried predominantly by the electromagnetic field. Here, we describe recent theoretical work on the formation of relativistic Poynting jets from magnetized accretion disks. Further, we describe new relativistic, fully electromagnetic, particle-in-cell (PIC) simulations of the formation of jets from accretion disks. Analog Z-pinch experiments may help to understand the origin of astrophysical jets.     

Evidence of the Link between Broad Emission Line Regions and Accretion Disks in Active Galactic Nuclei

1 INTRODUCTION Most bright active galactic nuclei (AGNs) exhibit broad emission lines, with full width at half maximum (FWHM ≥ 103 km s?1) (Peterson et al. 1999). Some type 1 AGNs could have very broad emission lines (FWHM≥ 20 000 km s?1). Type 2 AGNs s…     

Star Formation in Accretion Disks Around Massive Black Holes and Pregalactic Enrichment

Broad Absorption Lines (BALs) prove the existence of a high velocity outflowing gas with metallicities larger than solar in the central few parsecs of high redshift quasars. At the same distance from the black hole, accretion disks in quasars and Active Galactic Nuclei (AGN) are locally gravitationally unstable, and clumps must form with a size of the order of the scale height of the disk. This is hardly a coincidence, and we have tried to link these two facts. We have assumed that the unstable clumps give rise to protostars, which become massive stars after a rapid stage of accretion, and explode as supernovae, producing strong outflows perpendicular to the disk and inducing outward transfer of angular momentum in the plane of the disk. As a consequence a self-regulated disk made of gas and stars where supernovae sustain the inflow mass rate required by the AGN is a viable solution in this region of the disk. This model could explain the BALs, and could also account for a pregalactic enrichment of the intergalactic medium and of the Galaxy, if massive black holes formed early in the Universe. This revised version was published online in July 2006 with corrections to the Cover Date.     

Turbulence in Accretion Disks

An accretion disk is an inevitable part of the star forming process. Recent years have witnessed dramatic progress in our understanding of how turbulence arises and transports angular momentum in astrophysical accretion disks. The key conceptual point is that the combination of a subthermal magnetic field and outwardly decreasing differential rotation is subject to the magnetorotational instability. This rapidly generates magnetohydrodynamical (MHD) turbulence, leading to greatly enhanced angular momentum transport. Purely hydrodynamic disks, on the other hand, are stable. Disks that are too cool to couple effectively to the magnetic field will not be turbulent. Fully global three dimensional MHD simulations are now beginning to probe the properties of accretion disks from first principles.     

Line Emission from Conical Jets Around Rotating Black Holes

Some observations indicate the presence of Doppler-shifted emission lines in the X-ray spectra of black hole candidates and active galactic nuclei. Motivated by which, we studied the observed profile of emission lines from relativistic outflows around black holes in great detail, but jets are simply treated as pencil beams. It was found that the observed line emission shows a doubly peaked profile at large viewing angles for extreme Kerr black holes. In this work, we study the observed profile of an emission line from a more realistic, cone-shaped jet, and we find that an intrinsically narrow line is basically singly peaked, which is different from our previous results. Therefore, we conclude that the double peaks in our previous results is due to the simplified assumption on the geometrical structure of jets. This finding emphasizes the difference between the observed line profile in the jet case and those in the disc case in which there are generally double peaks in the observed spectra.     

Periodic Orbits and Accretion Disks

The effect of the radiation pressure on the periodic motion of a small particle around one ‘primary’ body of the restricted photogravitational three-body problem is examined. Simple periodic orbits are used to determine the maximum size of the accretion disk in close binary systems. This revised version was published online in July 2006 with corrections to the Cover Date.     

Accretion and Ejections in Magnetized Disks

In this contribution I review the main challenges for theory and numerical simulation of accretion turbulence in disks. I then present briefly a solution we have elaborated in recent years to a part of these questions: we have found an MHD instability which occurs in the inner region of a disk in the configuration (poloidal field of the order of equipartition with the gas pressure) used for MHD jet models. This instability has the unique characteristic that it re-emits toward the corona a fraction of the energy and angular momentum it extracts from the disk. It is a good candidate to explain the low-frequency Quasi-Periodic Oscillation observed in X-ray binaries, and we believe that it might occur also in YSO.     

Emission Line Profiles of Warped Disks in the Kerr Spacetime

The calculations of emission line pro?les of a warped disk in the Kerr spacetime have been discussed in this paper. The main content consists of two parts. In the ?rst part, the geodesic motion in the Kerr spacetime and the equations of motion in integral form, as well as the method to obtain the solution with the Weierstrass’ elliptic functions are presented. Using the Weierstrass’ elliptic functions, the Boyer-Lindquist (B-L) coordinates and the affine parameter o are semi-analytically expressed as the functions of a parameter p. Then a code named ynogk (Yunnan Observatory Geodesic Kerr) is introduced based on the above discussions to calculate very fast the null geodesic in the Kerr spacetime. As an application of ynogk, the emission line pro?les of warped disks are investigated in detail in the second part of this paper. Here the structure model of warped disks is based on the results of Bardeen and Petterson in 1975, and the line pro?les are computed with the ray tracing method. Finally, the discussions and conclusions of the calculated results are presented. It is indicated that the line pro?les are dependent mainly on the inclination and azimuthal angles of the observer, as well as the index of emissivity, and that they may have a triple-peaked even multi-peaked structure, different from the double-peaked structure in the line pro?les of a standard thin accretion disk.     

黑洞吸积盘的数值模拟

近十几年来,人们已经对吸积盘进行了深入的研究。前人很多工作都是解析工作。虽然解析工作能够简单明了地揭示盘的许多重要性质,但吸积盘的很多重要性质(例如:对流和外流)没有办法通过解析工作进行深入的研究。数值模拟是研究天体物理的重要手段。近十年来,随着计算机技术的迅速发展,数值模拟在天体物理研究中扮演了越来越重要的角色。本论文主要用数值模拟的手段研究吸积盘中一些重要的物理过程。     

Magnetic Interaction Between Stars And Accretion Disks

In this review I consider modern theoretical models of coupled star–disk magnetospheres. I discuss a number of models, both stationary and time-dependent, and examine what physical conditions govern the selection of a preferred model.     

Disks Around Post-Main Sequence Binaries

We propose that at least two stars on or near the AGB have long-lived orbiting disks: HD 44179, the central star in the Red Rectangle, and BM Gem, a carbon-rich star with an oxygen-rich circumstellar envelope. The CO emission from both of these disks has a spike with a width near ∼2 km s⁻¹, indicating disk radii of ∼10¹⁶ cm. The dust in such disks is therefore quite cold (near T ∼ 50 K for the Red Rectangle) and may emit primarily at submillimeter wavelengths. The disks around stars where there is also substantial mass loss may not be easily observable; there could be many as yet undiscovered disks around AGB stars This revised version was published online in September 2006 with corrections to the Cover Date.     

磁化吸积盘的不稳定性研究

从磁流体动力学方程组出发,用微扰法得到的色散方程中含有环向磁场.利用全新的反常粘滞和反常阻抗,对吸积盘进行数值计算,结果表明,只有竖直方向的弱磁场才可以引发一种单调不稳定性.磁场对粘性吸积盘表现为非稳定性因素,增长率随磁场的增强而增大,且最大增长率大于理想情况下的值.垂向磁场足够强时,单调不稳定性不会出现.     

黑洞吸积盘理论进展

在已知的四种黑洞吸积盘模型中，Shapiro-Lightman-Eardley(SLE)盘是不稳定的，细（Slim)盘研究得还不够，较为成功的是最早建立的Shakura-Sunyaev盘（SSD）和近年成为热点的径移主导吸积流（ADAF）。SSD和ADAF看来分别适用于吸积流中离黑洞较远和较近的区域，故二者的结合即ADAF＋SSD模型有望对黑洞吸积流作出较为完整的描述，但这个结合模型也还有不少未解决的问题。     

含粘滞和磁场的薄吸积盘不稳定性

本文采用微扰方法导出色散方程，并在四种情况下详细讨论了薄吸积盘的不稳定性。结果表明：在纯粘滞和纯磁场盘中都存在脉动不稳定性。而且在吸积盘内同时考虑粘滞和磁场时，存在两种不稳定性，一种是脉动不稳定性，另一种是单调不稳定性。同时数值计算还表明，脉动不稳定性更可能存在于盘的内区，而单调不稳定性则只在盘的外区，对短波扰动才有意义。这些结果为解释ＢＬＬａｃ天体、Ｓｅｙｆｅｒｔ星系及类星体等活动星系核的光变现象进一步提供了理论依据。     

奇异星周围的中微子主导吸积流

为统一解释伽玛射线暴(简称伽玛暴)与暴后再活动,提出了一个新的伽玛暴中心引擎模型一“奇异星-NDAF”模型(NDAF:Neutrino Dominated Accretion Flow,中微子主导吸积流),并计算了奇异星周围NDAF的结构.与其他中心致密天体不同的是,奇异星会向吸积流反馈以中微子为载体的奇异化相变能量.不考虑NDAF与奇异星的摩擦,结果表明:奇异星周围NDAF的结构对吸积率非常敏感;当吸积率大于0.18 Mo.S-1时,“奇异星-NDAF”模型能统一解释伽玛暴与暴后再活动,这个范围大于无摩擦的“中子星-NDAF”模型能统一解释的范围;在统一解释的情形下, “奇异星-NDAF”模型湮灭总能量的分布非常宽阔,当吸积率大于0.3 M0.S--1时,湮灭总能量大于1051 erg;最后,当吸积率大于0.3 M0.S-1时,“奇异星-NDAF”模型的湮灭光度超过同等吸积率下“黑洞-NDAF”模型一个多量级,有利于解释某些光度极大的伽玛暴.