节点文献
同步曲率辐射机制的物理特性
【作者】 刘炯；
【作者基本信息】 中国科学技术大学 ， 天体物理， 2006， 博士
【摘要】 同步-曲率辐射的提出解决了弯曲磁场中相对论性粒子的辐射计算问题，并且成功的用于包括脉冲星，伽玛暴(Gamma-Ray Bursts，GRBs)以及AGNs在内的许多问题的研究中。本论文根据同步-曲率辐射理论，进一步拟合了一些伽玛暴谱，分析了一般弯曲磁场中带电粒子的集体辐射特征，研究了等离子体环境中的同步-曲率辐射的脉泽现象，并且讨论了可能的应用。同步-曲率辐射使自洽地研究天体物理各类磁场中相对论性带电粒子的辐射成为可能。在本论文的第一章，我们详细介绍了同步-曲率辐射的基本特性。为了完整起见，在第一章也简要的回顾了同步辐射，曲率辐射的一些主要的内容，然后综述了同步-曲率辐射理论。磁场中幂律能量分布电子的辐射在天体物理研究中具有极端重要性，但是同步-曲率辐射机制则指出这样的辐射谱不再是单一幂律谱，其低频谱接近同步辐射幂律谱，高频谱近似接近曲率辐射幂律谱，两者光滑地相接。这一高频谱变平的性质是弯曲磁场的效应，只有同步-曲率辐射机制可以很好的对其加以解释。弯曲磁场广泛存在于各类天体中的实际磁场，特别是激波中形成的磁场，虽然目前尚难以了解其形成过程，但用均匀场处理则肯定不真实。非均匀磁场中的同步-曲率辐射公式有助于处理这一问题。在1998年，袁业飞和张家铝成功的将同步-曲率辐射机制量子化，使得整个同步-曲率辐射理论在物理上显得完整可靠。在论文的第二章我们简要的叙述了脉冲星磁层中的高能辐射过程，介绍了现在得到大多数学者认可的三种脉冲星外层辐射模型(1)极冠模型，(2)外间隙模型，(3)狭长间隙模型。虽然目前人们对脉冲星的辐射机制认识还很不深入，还没有一种模型能够统一地解释脉冲星多波段的观测特性(e．g．Michel 1991)，但是所有模型中都有一个共同点，那就是在快速旋转的中子星磁层中引入一个强电场，而且脉冲星的磁场位型是建立在弯曲的偶极磁场位型的基础之上。因此，当人们使用同步辐射或者回旋辐射机制研究脉冲星的外层辐射时就会产生偏差，而同步-曲率辐射机制可以较好的解决磁场弯曲所带来的影响。在论文的第三章，我们以现在广为接受的火球激波模型为主简要的介绍了伽玛暴现象。伽玛暴是宇宙空间中的一种短时标的伽玛射线爆发现象，它的大部分辐射能量在30KeV到10MeV之间，小部分能量以余辉形式释放。对伽玛暴的研究早已成为天体物理的热点之一。使用辐射理论对伽玛暴的辐射谱进行研究有利于了解伽玛暴的辐射机制，进而对伽玛暴这种神秘的现象本身的起因，形成机制，能量来源等重要问题的研究提供极其重要的帮助。第四章到第六章为本论文的主要研究工作，在第四章中我们分析了火球激波模型的不足之处，指出火球激波模型虽然能从整体上形象地说明了爆发后的过程和主要特征，但模型认为激波中的辐射最初都来源于同步辐射，这可能就是一个不足之处，并且因此造成其在解释某些最新的观测结果时遇到困难。我们发现同步-曲率辐射机制可以比较好的拟合一系列伽玛暴谱，并且给出了一些例子，很好地解释了过去使用同步辐射机制拟合时无法解释的高能拐点和能量过剩现象，这一结果很有说服力的表明伽玛暴的磁场不可能是同步辐射所需要的均匀平直磁场，使用同步-曲率辐射机制可能是一个合适的选择。这一系列拟合结果说明同步-曲率辐射机制可以为GRB的高能伽玛光子提供一种新的值得参考的解决方案，而且它也促使我们思考一些更加深层次的问题，特别是磁场结构，激发机理，物理特性等。我们首次给出了可能的磁场位型和产生机制，这是使用同步辐射模型无法考虑的。对于激波的加速机制，我们通过对拟合结果的分析找到了限制条件，推动了对加速机制的认识和研究，也从一个侧面说明了同步-曲率辐射机制是一种有广阔应用前景的辐射机制，对于弯曲磁场的辐射它是必需的。在第五章，我们给出了等离子体环境下同步-曲率辐射的自吸收系数并且研究了同步-曲率的脉泽现象。如果不考虑曲率漂移，Blandford(1975)已经证明曲率脉泽发射是不可能的。我们以前的工作证明如果不考虑漂移(漂移效应将在第六章中讨论)，同步-曲率辐射在真空中不会有脉泽发射(Yang et al 2003)。虽然在真空中同步-曲率辐射与同步辐射一样没有负吸收放大作用(即脉泽效应)，但等离子体对辐射谱和自吸收性质有重大影响，在考虑活动星系核等天体中等离子体的辐射问题时必须考虑。我们的计算表明在等离子体环境中完全可能出现负吸收放大效应。由于同步-曲率辐射功率要显著高于同步辐射功率，同步-曲率辐射脉泽效应可能远强于同步辐射脉泽效应，这有助于解决宇宙中多种高亮射电源的温度等疑难。在第六章，我们研究了粒子在弯曲磁场中运动的漂移效应，我们发现考虑到引导中心的漂移特性后，脉泽可以在真空中产生。如果不考虑漂移作用，单纯的同步-曲率辐射机制给出的结果是理想化的，因为沿着弯曲的磁力线运动的带电粒子必然受到某种力，并且将导致运动路径的偏离，在弯曲磁场中的作螺旋运动的带电粒子的引导中心必然要发生漂移，并且对辐射谱产生不可忽略的贡献(Harko & Cheng 2002)。我们前面已经指出GRB磁场可能是弯曲的，根据前几章的讨论，同步-曲率辐射机制在GRB中的应用是值得重视的，同时考虑漂移效应，利用我们推导出来的关系，我们认为GRB中的ξ_e(加速相对论电子的总能量与激波动能之比)和ξ_B(激波中的磁场的能量密度与激波动能之比)将会和单纯考虑同步辐射时的情况相比有更强的限制，因此将为解决GRB的前身星和发动机问题提供更加仔细的线索。
【Abstract】 The synchron-curvature mechanism solves the radiation problem of relativistic particles moving in a curved magnetic fields. This mechanism has already been applied in many astrophysics sources including pulsar, GRBs and AGNs. In this paper, according to the synchron-curvature radiation theory, we successful fit the spectrums of GRB, and discuss the radiation properties of the charged particles moving in a gerenal curved magnetic fields. Then we study the maser phenomenon under plasma condition also including the effects of the drift of charged particles. Finally we discuss some potential applications of these results.It is possible to research the radiation properties of the relativisitic charged particles precisely in astrophysical magnetic field, since the synchron-curvature radiation mechanism was discovered. In Chapter 1, we briefly go over the synchrotron radiation mechanism and curvature radiation mechanism, then we introduced the synchro-curvature radiation mechanism. With respect to the radiation of power law energy distribution of electrons, which is of great importance in astrophysics, we demonstrate that their synchro-curvature radiation spectrum is no longer simple power law. The low frequency and high frequency segments are approximately similar to that of synchrotron and curvature process respectively, and both segments are connected smoothly. The flatness in the higher frequency spectrum is due to the effect of the curved magnetic field lines and can well interpret the abnormal continuum in their spectra observed in some cosmic sources. Regarding the realistic magnetic fields widely exist in various cosmic sources, especially that formed behind the shock, although we still have no any idea of the formation process, the treatment in the uniform fields is definitely not correct. Our formula for the non-uniform fields are helpful for studying the problem. In 1998, Yuan and Zhang obtained the quantized formula of synchron-curvature radiation mechanism, these quantized formula is an important development section of the whole synchron-curavture theory.In Chapter 2, We also briefly introduce the high energy radiation process of pulsars and three famous radiation models for γ ray pulsars: (1)Polar Gap Model, (2)Out Gap Model, (3) Slot Gap Model. Although many kinds of pulsar model are different with each other and none of them can explain the radiation from the 7 ray pulsars completely, but almost all kinds of models have a common characteristics, for example, a strong electric field exists in the magnetphere of pulsars and the magnetic field is assumed to be a curved dipole magnetic field. So it will be not precise if we discuss the radiation process of pulsars with synchrotron radiation or cyclotron radiation, but the synchron-curvature radiation can solve the curvature effect of the magnetic field very well. In Chapter 3, we give a brief introduction to GRB physics, especially the well-accepted fireball-shock model. GRB is a phenomenon of a short time scale burst of Gamma ray. Most of the radiation energy of the GRB is between 30Kev and 10Mev, some small amount of energy powers their afterglow. By analysing the spectrum of the GRB, we can understand the radiation mechanism of GRBs and the central engine of GRBs.Our works are included in Chapter 4-6. In Chapter 4, we discuss the fault of the fireball shock model. Although this model can demonstrate the gerenal process of the GRBs and can explain most of the properties of the GRBs’ afterglow, but in this model, the radiation is generally regarded as synchrotron radiation. This viewpoint may be wrong and may be cause the recent difficulty of explaining the update observations. We discovered that the synchron-curvature radiation mechanism can fit a series of spectra of GRBs very well. We can explain the high energy inflexion and energy surplus which can not be explained by synchrotron mechanism. This means that the magnetic fields of GRBs is curved and can not satisfy the requirements of synchrotron radiation. So the synchron-curvature theory is a good choice for GRBs. According to our results, we establish a possible magnetic field form of GRBs and discuss the formation of this magnetic field form. After we analyse the spectrums of GRBs which is fitted by synchron-curvature radiation mechanism, we find out a constrain of the acceleration of the shock of the GRBs. All these things help us realize that the synchron-curvature radiation mechanism can be widely applied in astrophysics.In chapter 5, we research both the self-absorption properties of the synchron-curvature radiation and the maser phenomenon. Neglecting the curvature drift (we will talk about the effect of drift in Chapter 6), Blandford(1975) had proved that the curvature maser emission is impossible. It has been found out that maser is impossible in vacuum (if the effects of the drift of the guiding center are neglected (Yang 2003)). We find that although the synchron-curvature process has no maser action in vacuum, the effects of plasma can cause maser action according to our theorectical analysis and calculations. As the power radiated by synchron-curvature mechanism is substantially greater than that of synchrotron one, the maser of the synchron-curvature mechanism could be more powerful than that of the synchrotron one, which maybe helpful for the solution of the extreme temperature trouble in various radio sources.In chapter 6, we consider the drift effect of the charged particles, which is moving along the curved magnetic field. Including the drift of their the guiding center, we show that the maser emission is possible even in vacuum. The unified synchro-curvature mechanism(Zhang&Cheng 1995; Cheng&Zhang 1996), if without drift, can be developed further because such kind of magnetic field will create a magnetic gradient, which gener- ates some kinds of force and then cause the drift motion of the particles. Sometime, this effect can give a non-negligible contribution to the radiation spectrum (Harko&Cheng 2002). As the magnetic field of GRBs is curved, if we consider the synchro-curvature maser including the drift, we are sure that the values of the ξ_e and ξ_b will be different from the ones if only the synchrotron radiation is considered. These effects may serve as an important clue to the GRB environment and their progenitor.