【作者】 刘当波；

【导师】 尤峻汉；

【作者基本信息】 上海交通大学 ， 理论物理， 2006， 博士

【摘要】 最近十几年来，由于空间技术的飞速发展，使X射线和γ射线天文学进入空前繁荣时期，极大地扩展和深化了我们对宇宙，对自然界的认识。各种新发现，新理论，新学说层出不穷，成果举世瞩目。例如，活动星系核（AGN），特别是Seyfert 1星系的性质奇特的铁Kα线的发现，就是近十年X射线天文学最重要的成果之一。铁Kα线是认识AGN中央大黑洞周边物理环境及状态的最重要探针，因此成为近年X射线天文观测和黑洞理论研究的一大热点。再如近十年中伽玛暴研究的巨大进展，使之三次入选美国‘Scicence’杂志评出的年度十大科学成就。伽玛暴研究已经成为天体物理学中一个焦点领域，对今后物理学和天文学发展有很大的潜在意义。人们期待着，新的科学突破会从天文学开始。但与观测成果相比，理论现状相对落后。这包括两方面：即对这些高能天体的物理了解及对高能辐射机理本身的基础研究。弄清辐射机理是揭示这些奇异天体性质的关键第一步，而目前国际上在这方面进展不大。仅对活动星系核，类星体，以及伽玛暴这几类天体而言，面对观测资料，至今存在着很多无法解决的困惑。有些难题甚至已经困扰天文界二、三十年。理论落后观测的现状首先表现在辐射理论研究进展缓慢，而目前新发现的高能天体的观测性质对经典辐射理论提出了严重挑战。为此，本论文的主要研究课题是X射线和γ射线天文学中新辐射机制的研究及其应用。第一章为绪论，概述了本文对辐射机制研究的主要内容。从第二章到第十一章主要对Cerenkov线状辐射理论，强磁场中回旋辐射的量子理论，强磁场中相对论电子的共振逆Compton散射机制（简称RICS），Compton软化处理方法和双光子湮灭吸收及其它们在天体物理中的应用进行了一系列的研究．现分叙如下。在上世纪80年代，尤峻汉等人就提出，当具有各向同性速度分布的相对论电子穿过气体区域或者轰击稠密气体区域的表面时，所产生的Cerenkov辐射集中在原子特征谱线附近的狭窄频段上，因而这种辐射可看作一种特殊的发射线，称为Cerenkov线状辐射或简称Cerenkov发射线。这是一种新的谱线发生机制。在第二章中简介Cerenkov线状辐射基本理论，系统地引出Cerenkov线状辐射的相关公式。定量给出Cerenkov出射线的强度、轮廓、线宽、红移等特征。这种新型谱线发射机制在天体物理中有潜在的重要性，可用来探讨活动星系核的宽发射线的起源和性质（包括类星体和Seyfert 1星系在X射线和光学波段的宽线）。本章中，我们利用Cerenkov线状辐射理论公式计算了氢原子巴尔末谱线的强度比，成功地解释了类星体与赛弗特Ⅰ型星系中很陡的巴尔末减缩。另外，我们也利用Cerenkov线状辐射理论成功地解释了类星体的氢发射线之间系统性的微小红移差。这为活动星系核的氢的宽发射线主要起源于Cerenkov线状辐射机制的提供了观测证据。铁K线是目前公认的认识活动星系核（ACN）中央大黑洞和X射线密近双星黑洞周遍物理环境及状态的最重要的探针。第三章中，利用Cerenkov线状辐射机制探讨了活动星系核中的铁K线起源问题。为解决目前流行的“光电吸收-荧光线发射”机制的某些预言与观测的不符，我们建议采用Cerenkov线发射机制重新探讨AGN中的铁K线起源。理论计算表明，在合理的参数范围内，的确可以产生足够大的能与观测比较的Cerenkov谱线强度（或光度）。我们利用Cerenkov机制也合理地解释了观测中反常的铁线Kα／Kβ强度比。并预言铁Kα线和Kβ线存在系统性的红移差，期待观测上的验证。我们这一新模型的提出既为解释AGNs中的铁K线提出了新的途径，同时也对AGNs的物理研究有明显的影响，AGN中心巨黑洞的周边环境可能比以前所设想的有更多的激烈高能活动，有大量相对论电子存在，而且气体也更加稠密。在X射线和γ射线天文学中，强磁场中非相对论性热电子的回旋辐射是一种重要的谱线发射和谱线吸收机制。在强磁场中，经典电磁学给出的回旋辐射理论已不再适用。本文第四章中，我们介绍了尤峻汉等人近年发展的回旋辐射的半经典量子理论。与以前的QED结果相比，回旋辐射的半经典量子理论的结果更具物理直观性，且更为简洁实用。本章中，我们利用量子回旋吸收公式对孤立中子星1E1207.4-5209 X射线回旋吸收线的做了理论分析，完成了对位于0.7和1.4 keV两条吸收线的等值宽度的拟合，并由此导出自转轴的空间取向。强磁场中相对论电子的共振逆Compton散射（简称RICS）是γ射线天文学中一种重要的γ射线辐射机制。与一般逆Compton散射不同，RICS机制讨论强磁场中相对论电子沿中子星磁轴飞行的共振的逆Compton散射。这是一种新的高效、高能、辐射带有尖锐方向性的辐射机制。在第五章，我们给出单个相对论电子共振逆Compton散射的解析谱功率分布公式，给出了实验室系中共振逆Compton散射的匹配条件γhv_i（?）hv_B（或（?）h（?）_i（?）hv_B，又叫近似共振条件）。当幂律分布的N（γ）=N₀γ^-n（γ₁＜γ＜γ₂）的电子系集体沿磁偶极场运动时，我们计算了变化磁场中集体的共振逆康普顿散射（RICS），得到了各种典型的周边低频场环境下的集体RICS谱形。在5.5节中，我们尝试利用RICS机制重新探讨伽玛射线暴中原初伽玛射线的辐射。指出了流行的‘火球—内激波’模型在解释伽玛射线暴中原初伽玛射线辐射存在的一些问题，论证了RICS机制产生伽玛射线暴中原初伽玛射线辐射的可能性，RICS机制可能是解释伽玛射线暴中早期伽玛射线辐射的有希望的新尝试。X射线穿过‘冷’等离子体的Compton软化过程是X射线天文学中辐射转移中的一个重要内容。由于辐射场和等离体介质的能量交换，将会使出射光的谱形发生很大的改变。第六章中介绍处理X射线或γ射线穿过‘冷’等离子体的Compton软化过程的一种简便的解析方法—推广的Kompaneets扩散方程。该方程既适用于Compton软化过程，也适用于Compton硬化过程。利用此推广的Kompaneets方程计算了天体物理中4种常见辐射谱（发射线、黑体谱、幂律谱和热轫致谱）的Compton软化，所得结果同MonteCarlo模拟结果和Ross-McCray方程的数值求解结果一致。相对于Monte Carlo方法，推广的Kompaneets扩散方程更具物理直观性，且求解简捷方便。因此，推广的Kompaneets方程是处理Compton软化的一种有效方法。双光子湮灭造成的γ射线吸收是γ射线天文学中一种重要的吸收机制。在第七章，我们对双光子湮灭造成的γ射线吸收做了详细的理论分析，得出了双光子湮灭的匹配条件（hω）·（hω′）≈3（m₀c²）²，有利于对伽玛射线观测谱做定性物理分析，预言了在宇宙伽玛辐射谱中可能会出现线状吸收结构。另外，我们也得到了不同低频辐射场下双光子湮灭的吸收系数，可用来定量分析伽玛辐射谱穿过周边低频场所造成的吸收。总之，在本篇论文中，主要阐述了我攻博期间在X射线和γ射线天文学中辐射机制的理论和应用研究方面所做的研究工作。研究工作主要包括：Cerenkov线状辐射理论的应用研究，强磁场中回旋辐射量子理论的应用，强磁场中相对论电子的共振逆Compton散射机制的研究及应用，Compton软化处理方法和双光子湮灭吸收。在第八章中我们对本论文的主要研究内容做了一个简单总结，并对我们以后在X射线和γ射线辐射机制方面可能的应用研究做了一些展望。更多还原

【Abstract】 In the past decades, owing to the rapid development of the space technology, the X-ray and γ-ray astrophysics has got a big progress and is thriving as never before, which greatly expands and deepens our understanding on the universe. Various new discoveries and theories in this area present in recent years and attract people’s attention. For example, the successful detection of the iron Kα line emission at ～ 6.4 keV in Seyfert 1s is undoubtedly one of the most striking achievements in X-ray astronomy. It is a consensus that the iron Kα line is a powerful probe for understanding of the environment around the central super-massive black hole of the active galactic nuclei （AGNs）. Therefore the study of iron Kα line has been a hotspot in both the X-ray observation and the theory of black hole. Moreover, the new discovery and understanding of the gamma ray bursts （GRBs） are the most important advances in the γ-ray astronomy in recent years. The breakthroughs in GRBs study have been elected into the journal ’Science’ as one of the ’ten annual events in the world science’ for three times, i.e., in 1997, 1999, and 2003, respectively. Owing to the potential importance for promoting the development of the modern physics and astronomy, now the GRBs research becomes a focus area in the contemporary astrophysics. People expect that new breakthroughs in science would occur in the astronomy.However, comparing with the achievement in observations, the progress in the theoretical researches is somewhat lagged. This embodies in both the lack of understanding for these newly discovered high energy objects themselves and the insufficiency of the fundamental investigations of the high energy radiation mechanisms. We claim that the key step of exploration of these strange objects is to clarify the mechanisms of the high energy radiation in the γ-ray and hard X-ray wavebands. But this is just a weakness point in this research area. Take AGNs and GRBs as an example, facing abundant observation data, people still have a number of puzzles to be resolved. Some of them even have perplexed astronomers for several decades. At present, the observational properties of these newly discovered objects raise a serious challenge to the conventional radiation theory. Therefore, I devote myself to the research of the radiation theory. The main topic of this dissertation is concentrated on the theoretical investigations of the radiation mechanisms and its applications in the γ-ray and X-ray astrophysics. The arrangement of my thesis is as follows: Chapter 1 is an introduction, where I outline the main contents of this thesis describing my outcomes in researches. From Chapter 2 to Chapter 7 I present the detailed descriptions of new radiation mechanisms and their applications in the high energy astrophysics, including the Cerenkov line-like radiation theory, the quantum theory of cyclotron radiation, the resonant inverse Compton scattering （RICS） in an intense magnetic field, the analytical methods for dealing with the down-Comptonization and the further analysis of the two-photon annihilation absorption, etc.In 1980’s, You and Cheng suggested that, when the thermal relativistic electrons with isotropic distribution of velocities move through a dense gas region, or impinge upon the surface of a cloud with dense gas, the Cerenkov effect will produce peculiar atomic or ionic emission lines. We call it ’Cerenkov line-like radiation’ or simply ’Cerenkov emission line’. The basic principle of the Cerenkov line-like radiation given by You et al. is summarized in Chapter 2, including the relevant basic formulae describing the intensity, profile, line-width and small redshift etc. of the Cerenkov line. We mention that the newly recognized line emission mechanism has a potential importance in the high energy astrophysics, e.g., in the exploration of the origin and properties of the optical and the X-ray broad emission lines in AGNs. In this Chapter, by using the new mechanism, we successfully complete a model calculation of the intensity ratios of Balmer lines of hydrogen to successfully fit the observed steep Balmer decrement of quasars and Seyfert Is, a long standing puzzle in the AGNs study in past three decades. The coincidence between theory and observations is unexpected excellent. Moreover, we successfully explain the slight redshift differences among broad hydrogen lines by use of the same mechanism. Both the two successfully provide strong evidences supporting that the broad hydrogen lines of AGNs arise from the newly recognized Cerenkov line-like radiation.It is a consensus that the iron Kα emission line is a powerful probe for exploring the physics of the environment around the central superniassive black hole of AGNs or the stellar black hole of X-ray binaries. In Chapter 3, we explore the problem of origin of iron Kα lines in AGNs. So far the prevailing fluorescence-recombination model for the Fe Kα line suffers some serious difficulties confronting with the observations of AGNs. Some predictions of the traditional model are markedly inconsistent with observations. We therefore attempt a new solution by using the Cerenkov linelike radiation for these puzzles. The calculated Cerenkov iron K-line is strong enough to match observations. The problems of the anomalous intensity-ratio of iron Kα/Kβ and lack of response of iron K-line flux to the continuum variability, etc., can be solved by this way. Undoubtedly, such a new suggestion for the iron K-lines origin will significantly change our understanding of the physics of AGNs. If the Cerenkov origin of the iron K-lines is further confirmed by future observations, the conventional scenario of the environment around the central superniassive black hole would be replaced by a frequently changed, more violent, more energetic picture with abundant relativistic electrons and much denser gas regions.The cyclotron radiation in a strong magnetic field is an important line emission or absorption mechanism in the X-ray and 7-ray astronomy. However, the classical cyclotron theory is no longer valid in the strong magnetic field with B > 10⁸ E_eV, which must be replaced by the quantum theory. In Chapter 4, we briefly introduce the semi-classical quantum theory of cyclotron radiation under the quadrupole approximation, given by You and Chen in 1997. Comparing with the QED results, our semi-quantum theory is more simple and lucid with a physical intuition, more convenient in the practical application. As an example, we analyze the observed absorption features of the isolated neutron star IEI207.4-5209 regularly spaced at 0.7, 1.4 and 2.1 keV, by using the semi-quantum cyclotron absorption formulae. We successfully fit the observed equivalent width of the absorption lines at 0.7 and 1.4 KeV, and obtain the direction of the spin axis in space of IEI207.4-5209.The resonant inverse Compton scattering （RICS） of the relativistic electrons in an intense magnetic field of a magnetized neutron star is an efficient radiation mechanism for producing the high energy γ-rays due to its high efficiency, high frequency, highly beaming behavior and comparatively good monochromaticity, concentrating most radiation in the high-frequency band. Differing with the ordinary Compton scattering of the free relativistic electrons, RICS is a special kind of scattering with a resonance property, produced by the relativistic electrons moving along the magnetic axis of the magnetized neutron star. In Chapter 5, we present the analytical formulae describing both the RICS spectral and the total powers of a single relativistic electron with energy γ and moving in a constand field with a given strength B. We further give the ’matching condition’ γ · hv_i ≈ hv_B （another terminology is ’condition of approximate resonance’）, under which the RICS efficiency achieves to the maximum. We calculate the collective RICS spectra of relativistic electrons with power law form spectrum N（γ）dγ = N₀γ^-n （γ₁ <γ < γ₂）, moving along the magnetic axis of neutron star and passing through the ambient soft-photon field with various typical spectral forms. In sec. 5.5, we try to explore the origin of the primary γ-ray radiation of GRBs by using the newly recognized RICS mechanism. We mention some difficulties in the prevailing ’fireball-internal Shockwaves’ models of GRBs and suggest an alternative possibility of RICS model for understanding the origin of the γ-ray radiation in the early stage of GRB events. We argue that the new RICS mechanism is a hopeful way to recognize the origin of early γ-ray radiation of GRBs.The down-Comptonization, occurring in the propagation process when the hard X-rays pass through a ’cold’ plasma, is an important process of the radiative transfer in X-ray astronomy and in radiation physics. Owing to the energy exchange between the radiation field and the plasma, the emergent X-ray spectrum will be changed markedly. In Chapter 6, we introduce a convenient and analytical method developed by ourselves, to deal with the down-Comptonization process, i.e., the extended Kompaneets diffusion equation, which is valid for both the up- and down-Comptonization processes. We calculate the evolutions of some typical X-ray spectra （emission line, black body, power law form and thermal bremsstrahlung spectrum）. Our results are well consistent with that given by Monte-Carlo simulation and by the Ross-McCray equation. Beside of the simplicity and physical clarity, a remarkable advantage of our extended Kompaneets equation is less expensive in terms of computational time.The two-photon annihilation （γ — γ reaction） is an important absorption mechanism in γ-ray astronomy and γ-ray physics. In Chapter 7, we give a more detailed and deeper analysis on this absorption process. We therefore obtain a very useful result in the γ — γ annihilation absorption process, which we call ’matching condition’, i.e., the relation （hω） · （hω’） ≈ 3（m₀c²）². Under this condition the annihilation probability of two interaction photons hω and hω’ achieves the maximum. The ’matching condition’ is very useful for the qualitative analysis of the observed γ-ray spectra, and can be used to predict some line-like absorption features in the γ-ray observations. Moreover, we calculate the curves of the annihilation absorption coefficients k_γγ（hω） ～ hω for various low-frequency fields, which can be used for the quantitative analysis of the observed γ-ray absorption spectrum when γ-rays of a source pass through the ambient low-frequency radiation field.In summary, in this Ph. D. thesis, I mainly present my own researches on the new radiation mechanisms and their applications in astrophysics. The researches include the Cerenkov line-like radiation and its application in the exploration of broad lines origin in AGNs, the quantum theory of cyclotron radiation in a strong magnetic field and its interpretation to the absorption features of isolated neutron star IE 1207.4-5209, the resonant inverse Compton scattering in an intense magnetic field and its possible application in GRBs, the methods dealing with the down-Comptonization and the two-photon annihilation absorption. Finally, in Chapter 8, I give a brief summary of my researches in this area. I particularly discuss the plan and prospect of my future study on the radiation mechanisms in X-ray and γ-ray astronomy.更多还原