活动星系核中黑洞质量和吸积率分布研究

红移、中心黑洞质量和吸积率是活动星系核演化的重要参数.利用反响映射法计算了172个类星体和Seyfert星系样本的中心黑洞质量,并分析了中心黑洞的质量、红移、爱丁顿吸积率的分布,进而验证了从类星体过渡到Seyfert星系的演化.     

径移主导吸积盘的含时数值模拟

黑洞吸积被认为是活动星系核的能量来源,而研究黑洞吸积最主要的难题是要解释吸积气体的角动量是如何转移出去的.黑洞吸积盘理论提出通过粘滞力矩转移角动量.径移主导吸积盘模型(advection-dominated accretion flow,简称ADAF)是几何厚、光学薄的低吸积率吸积盘模型,它在解释低光度活动星系核的一些观测现象时获得了很大的成功.近年来,随着计算机技术的迅猛发展,数值模拟在天体物理研究中获得了广泛的应用,也取得了突破性的进展.主要工作是用数值模拟方法研究ADAF整体解作为初始条件模拟黑洞的吸积过程.     

黑洞吸积理论及其天体物理学应用的近期发展(Ⅰ)

黑洞吸积理论是天体物理学的一个基础理论,是认识许多高能天体系统,如活动星系核、黑洞X射线双星、伽马射线暴等的重要物理基础。该文评述了近年来黑洞吸积理论,尤其是径移主导吸积流(advection-dominated accretion flow,ADAF)模型及其变种的主要发展,并介绍该理论在银河系中心、低光度活动星系核、黑洞X射线双星等方面的应用。全文分为两篇,该文是第一篇,内容是关于黑洞的热吸积流理论及其在银河系中心的应用。     

黑洞在顺行吸积和逆行吸积中的演化特征

分别在薄盘和厚盘两种情况下,讨论吸积盘的中心黑洞在顺行积和逆行吸积中的演化特征,结果表明,黑洞的温度和熵在逆行吸积与顺行吸积中的演化特征很不相同;在顺行吸积中黑洞的温度单调减小,而在逆行吸积中黑洞的温度存在一个极大值;在吸积的全过程中,黑洞的熵始终是增加的,而且逆行吸积中黑洞熵的变化率要比顺行吸积中的黑洞熵的变化率大得多,此外还讨论了在吸积过程中,黑洞质量的相对变化与黑洞自转的相关性。     

黑洞吸积理论及其天体物理学应用的近期发展(Ⅱ)

黑洞吸积理论是天体物理学的一个基础理论,是认识许多高能天体系统如活动星系核、黑洞X射线双星,以及伽马暴等的重要物理基础.该文评述近年来黑洞吸积理论尤其是径移主导吸积流模型(advection-dominated accretion flow)及其变种的主要发展,并介绍该理论在银河系中心、低光度活动星系核、黑洞X射线双星等方面的应用.共分为两篇,该文是第2篇,内容是关于黑洞热吸积流理论在低光度活动星系核以及黑洞X射线双星方面的应用.     

NDAF的中微子湮灭模型解释GRB 090510爆发

围绕着恒星级黑洞的中微子主导吸积盘可以通过盘上发出的中微子湮灭为伽玛暴提供能量。对于黑洞超吸积系统,吸积可能引起黑洞特征的极大演化,这会进一步引起中微子光度的演化。考虑不一样的平均吸积率和初始黑洞参数,通过吸积系统的演化分析中微子湮灭光度和总的中微子湮灭能量随时间的变化。同时计算了短暴GRB 090510的中微子湮灭能量并与理论预测的结果比对,发现中微子主导吸积盘的中微子湮灭能量的理论预测值远高于观测值,意味着这种模型可能提供GRB 090510爆发所需要的能量。     

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。     

黑洞吸积理论的新进展(Ⅰ):ADAF吸积理论

黑洞的吸积是天体物理学中最重要的基础理论之一。近年来该理论取得了引人瞩目的重大进展,具体表现在两个方面。其一是根据黑洞吸积必定跨声速这一特性,提出在一定条件下吸积流中会出现激波,这可称为含激波的吸积理论;其二是基于对一种局域致冷机制-贮导（advection）致冷的作用的重新认识而建立的,称为ADAF理论。在吸积盘的光学厚度很小或很大两种情况下,粘滞产生的大部分热量没有像在标准薄盘模型中那样辐射出去,而是贮存在流体中随流体的径向运动进入黑洞。与标准薄盘模型相比,贮导吸积盘具有高得多的温度和大得多的径向速度,但角动量小于开普勒角动量,吸积致能的效率要低得多。     

Blazar黑洞自旋能量与红移相关性研究

黑洞自旋及其参量能提供黑洞合并及吸积的信息。从文献资料中收集了112个Blazar源,这些源包含了67个FR II射电星系(RG),11个FR II射电噪类星体(RLQ),27个核占优星系(CD)。通过样本数据研究黑洞自旋能量与红移的相关性。研究结果表明:(1)112个Blazar的黑洞自旋能量与红移存在相关性,尤其在爱丁顿磁场条件下(B=BEDD),黑洞自旋能量与红移的相关性最为明显;(2)FR II射电星系(RG)、FR II射电噪类星体(RLQ)、核占优星系(CD)的黑洞自旋能量在3种磁场条件下(B=BEDD,B=104G,B∝j)与红移的相关性强弱上存在差异,但总体趋势较为相似,均呈现正比关系;(3)黑洞自旋能量与红移的强相关性表明,黑洞自旋能量在一定程度上给出黑洞并合与吸积的信息。这些研究结果与其他人用其他方法获得的结果是一致的。     

黑洞热吸积流的理论研究

<正>活动星系核(AGN)、低光度活动星系核(LLAGN)以及X射线双星(XRB)系统中都存在黑洞吸积过程.黑洞吸积是人们理解相关天体的辐射、光变等现象的关键.本学位论文主要侧重于对目前非常流行的热吸积流(径移主导吸积流ADAF以及明亮热吸积流LHAF)的研究.该类吸积流是理解LLAGN以及处于硬态的XRB的基本理论模型.论文第1章详细介绍了相关的背景.首先讨论了天体环境中的黑洞及其所处的系统—AGN、     

Reprocessed emission line profiles from dense clouds in geometrically thick accretion engines

The central engines of active galactic nuclei (AGN) contain cold, dense material as well as hot X-ray-emitting gas. The standard paradigm for the engine geometry is a cold thin disc sandwiched between hot X-ray coronae. Strong support for this geometry in Seyferts comes from the study of fluorescent iron line profiles, although the evidence is not ubiquitously airtight. The thin disc model of line profiles in AGN and in X-ray binaries should still be benchmarked against other plausible possibilities. One proposed alternative is an engine consisting of dense clouds embedded in an optically thin, geometrically thick X-ray-emitting engine. This model is also motivated by studies of geometrically thick engines such as advection-dominated accretion flows (ADAFs). Here we compute the reprocessed iron line profiles from dense clouds embedded in geometrically thick, optically thin X-ray-emitting discs near a Schwarzschild black hole. We consider a range of cloud distributions and disc solutions, including ADAFs, pure radial infall and bipolar outflows. We find that such models can reproduce line profiles similar to those from geometrically thin, optically thick discs and might help alleviate some of the problems encountered from the latter. Thus, independent of thin discs, thick disc engines can also exhibit iron line profiles if embedded dense clouds can survive long enough to reprocess radiation.     

Stellar tidal disruption by a massive binary black hole

The effects of formation of a binary black hole in a dense star cluster are found to have significant effects on the dynamics of the cluster. Tidal destruction of stars captured into bound orbits during the formation of the black hole binary provide a sizeable source of very high temperature thermal radiation as well as a source of radially outward moving clouds of gas. The efficiency of subsequently accreted matter onto the binary components as an energy source is investigated and suggestive evolutionary models of the dynamics of the binary system are presented. Lifetimes of the system are shown to be compatible with contemporary estimates. It is suggested that the high-density cluster core provides a suitable environment for the operation of a number of models for the core of active galactic nuclei.     

X-rays from cusps of compact remnants near galactic centres

Compact remnants – stellar mass black holes and neutron stars formed in the inner few parsec of galactic centres are predicted to sink into the central parsec due to dynamical friction on low-mass stars, forming a high concentration cusp. Same physical region may also contain very high-density molecular clouds and accretion discs that are needed to fuel supermassive black hole (SMBH) activity. Here we estimate gas capture rates on to the cusp of stellar remnants, and the resulting X-ray luminosity, as a function of the accretion disc mass. At low disc masses, most compact objects are too dim to be observable, whereas in the high disc case most of them are accreting at their Eddington rates. We find that for low accretion disc masses, compact remnant cusps may be more luminous than the central SMBHs. This 'diffuse' emission may be of importance for local moderately bright active galactic nuclei (AGNs), especially low-luminosity AGNs. We also briefly discuss how this expected emission can be used to put constraints on the black hole cusp near our Galactic Centre.     

Relation between radio core length and black hole mass for active galactic nuclei

We explore the relation between the linear length of radio core and the central black hole mass for a sample of radio-loud active galactic nuclei (AGNs). An empirical relation between the size of the broad line region (BLR) and optical luminosity is used to estimate the size of the BLR. The black hole mass is derived from H β linewidth and the radius of the BLR on the assumption that the clouds in BLRs are orbiting with Keplerian velocities. A significant intrinsic correlation is found between the linear length of the core and the black hole mass, which implies that the jet formation is closely related with the central black hole. We also find a strong correlation between the black hole mass and the core luminosity.     

Magnetic confinement, magnetohydrodynamic waves and smooth line profiles in active galactic nuclei

In this paper, we show that if the broad-line region clouds are in approximate energy equipartition between the magnetic field and gravity, as hypothesized by Rees, there will be a significant effect on the shape and smoothness of broad emission-line profiles in active galactic nuclei. Linewidths of contributing clouds or flow elements are much wider than their thermal widths, because of the presence of non-dissipative magnetohydrodynamic waves and their collective contribution produce emission-line profiles broader and smoother than would be expected if a magnetic field were not present. As an illustration, a simple model of isotropically emitting clouds, normally distributed in velocity, is used to show that smoothness can be achieved for less than ∼8×10⁴ clouds and may even be as low as a few hundred. We conclude that magnetic confinement has far-reaching consequences for observing and modelling active galactic nuclei.     

Broad reprocessed Balmer emission from warped accretion discs

Double-peaked broad emission lines in active galactic nuclei are generally considered to be formed in an accretion disc. In this paper, we compute the profiles of reprocessing emission lines from a relativistic, warped accretion disc around a black hole in order to explore the possibility that certain asymmetries in the double-peaked emission-line profile which cannot be explained by a circular Keplerian disc may be induced by disc warping. The disc warping also provides a solution for the energy budget in the emission-line region because it increases the solid angle of the outer disc portion subtended to the inner portion of the disc. We adopted a parametrized disc geometry and a central point-like source of ionizing radiation to capture the main characteristics of the emission-line profile from such discs. We find that the ratio between the blue and red peaks of the line profiles becoming less than unity can be naturally predicted by a twisted warped disc, and a third peak can be produced in some cases. We show that disc warping can reproduce the main features of multipeaked line profiles of four active galactic nuclei from the Sloan Digital Sky Survey.     

Line emission from accretion discs around black holes: the analytic approach

The broad X-ray iron line observed in many active galactic nuclei spectra is thought to originate from the accretion disc surrounding the putative supermassive black hole. We show here how to perform the analytical integration of the geodesic equations that describe the photon trajectories in the general case of a rotating black hole (Kerr metric), in order to write a fast and efficient numerical code for modelling emission line profiles from accretion discs.     

Line Shapes Emitted from Spiral Structures around Symmetric Orbits of Supermassive Binary Black Holes

Variability of active galactic nuclei is not well understood. One possible explanation is existence of supermassive binary black holes (SMBBH) in their centres. It is expected that major mergers are common in the Universe. It is expected that each supermassive black hole of every galaxy eventually finish as a SMBBH system in the core of newly formed galaxy. Here we model the emission line profiles of active galactic nuclei (AGN) assuming that the flux and emission line shape variations are induced by supermassive binary black hole systems (SMBBH). We assume that the accreting gas inside the circumbinary (CB) disk is photo ionized by mini accretion disk emission around each SMBBH. We calculate variations of emission line flux, shifts and shapes for different parameters of SMBBH orbits. We consider cases with different masses and inclinations for circular orbits and measure the effect to the shape of emission line profiles and flux variability.     

Acceleration of particles in the vicinity of a massive black hole

Recent results of the gamma-ray Cherenkov astronomy definitely prove the existence of fast variability in the very high energy (V.H.E.) gamma-ray flux of some active galactic nuclei. The BL Lac PKS 2155-304 for instance showed variations down to a few minutes time scale. From standard light travel time argument, these variations put extremely strong constraints on the size of the TeV emitting zone, which has to be of the order of a few Schwarzschild radius, even for high values of the relativistic Doppler factor of the emitting jets. Such discovery is a challenge for particle acceleration scenarios, which have to imagine efficient acceleration processes at work in a very compact zone. Eventually, the immediate vicinity of the central black hole appears as the most conservative choice for the location of the TeV emission region of active galactic nuclei. In this paper, we propose a two-step mechanism for charged particle acceleration in the magnetosphere of a massive black hole surrounded by an accretion disk. Particles first gain energy by a stochastic process during the accretion phase. It is shown that effective proton acceleration up to energies 10¹⁷–10¹⁹ eV is possible in a low-luminosity magnetized accretion disk with 2D turbulent motion. The distribution function of energetic protons over energies is a power law function with typical index ≃−1. Here electrons are not very efficiently accelerated because of their drastic losses by synchrotron radiation. In a second time, part of the fast particles escape from the disk and are then entrained by the magnetic structure above the disk, in the rotating black hole magnetosphere. They thus gain additional energy by direct centrifugal mechanism, up to about 10²⁰ eV for the protons and to 10–100 TeV for the electrons when they cross the light cylinder surface. Such energetic particles can further radiate in the TeV spectral range observed by Cherenkov experiments as HESS, MAGIC and VERITAS. Energetic protons can produce γ-radiation in the energy band 1 GeV–100 TeV and above mainly by nuclei collisions with the disk matter, clouds, or ambient low energy photons. Energetic electrons can also reach the required spectral range by inverse Compton emission. However their acceleration is less efficient due to heavy radiation losses, and only gained by centrifugal process during the second phase of the whole mechanism we describe. Our present analysis would therefore favor hadronic scenarios for TeV emission of active galactic nuclei. It is tempting to relate long term variability over years of TeV active galactic nuclei to the first stochastic acceleration phase, which also provides the needed power law particle distributions, while short term variability over minutes is more likely due to perturbations of the second fast direct acceleration phase.     

Massive black holes in nuclei of galaxies and quasars

In the first part of the paper the known results on the gravitational interaction of a massive black hole with the surrounding stars in a galactic nucleus are discussed. The tidal disruption of stars in close encounters with a black hole is reviewed. Expressions for the flux of stars on a black hole are given, taking into account energy and angular momentum diffusion of stellar orbits. The scenario of star disruption and accretion of the released stellar matter is depicted. The growth of a black hole in a typical galactic nucleus on account of gas accretion from disrupted stars is discussed. A comparison with the upper limit to the luminosity of the nucleus of our Galaxy puts rather severe constraints on the mass of a hypothetical black hole at the galactic centre. Possible mechanisms preventing the formation and growth of black holes in normal galactic nuclei are discussed.The second part of the paper (Section 8) deals with the hypothesis that massive black holes are the primary energy sources in active galaxies and quasars. The luminosity requirements of bright quasars and weak Seyferts can probably be accounted for in such a model, but there are difficulties in explaining the intermediate range. Mass ejection from Seyferts and quasars is not a severe problem. The same applies to the spectrum. A much more serious objection is the observed periodic and quasi-periodic variability. Another unsatisfactory feature of this hypothesis is that one needs two different evolutionary tracks for quasars and active galaxies, and for normal galaxies.