Investigations On The Theories And Applications Of New Radiation Mechanisms In The High Energy Astrophysics

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.